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Simon Peyton Jones pushed to branch wip/T24984 at <a href="https://gitlab.haskell.org/ghc/ghc">Glasgow Haskell Compiler / GHC</a>
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<strong style="font-weight: 600;"><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/62ea54a56fb6a672397374bfa2361743cd6a2c15">62ea54a5</a></strong>
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<span> by Simon Peyton Jones </span> <i> at 2024-07-26T11:14:31+01:00 </i>
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1 changed file:
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compiler/GHC/Tc/Solver/InertSet.hs
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<a href="https://gitlab.haskell.org/ghc/ghc/-/commit/62ea54a56fb6a672397374bfa2361743cd6a2c15#b97557795e2f14fc492ff8c685b149c247b5e480"><strong style="font-weight: 600;">compiler/GHC/Tc/Solver/InertSet.hs</strong></a>
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<td class="diff-line-num unfold js-unfold old_line" data-linenumber="1057" style="width: 35px; color: rgba(31,30,36,.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">...</td>
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<td class="line_content match" style="color: rgba(31,30,36,.24); padding: inherit;" bgcolor="#fbfafd">@@ -1057,7 +1057,17 @@ Proving termination of the extended substitution T is suprisingly tricky.</td>
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<td class="old_line diff-line-num" data-linenumber="1057" style="width: 35px; color: rgba(31,30,36,.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">
1057
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<td class="new_line diff-line-num" data-linenumber="1057" style="width: 35px; color: rgba(31,30,36,.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">
1057
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<td class="line_content" style="padding: inherit;"><pre style='display: block; font-size: 14px; color: #333238; position: relative; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'> <span id="LC1057" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> So we must kick out the inert item, even though (fs>=fw). (KK2b) does this</span></span>
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<td class="old_line diff-line-num" data-linenumber="1058" style="width: 35px; color: rgba(31,30,36,.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">
1058
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<td class="new_line diff-line-num" data-linenumber="1058" style="width: 35px; color: rgba(31,30,36,.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">
1058
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<td class="line_content" style="padding: inherit;"><pre style='display: block; font-size: 14px; color: #333238; position: relative; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'> <span id="LC1058" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> by looking for lhs_w under type family applications in rhs_s</span></span>
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<td class="old_line diff-line-num" data-linenumber="1059" style="width: 35px; color: rgba(31,30,36,.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">
1059
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<td class="new_line diff-line-num" data-linenumber="1059" style="width: 35px; color: rgba(31,30,36,.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">
1059
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<td class="line_content" style="padding: inherit;"><pre style='display: block; font-size: 14px; color: #333238; position: relative; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'> <span id="LC1059" class="line" lang="haskell"></span>
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<td class="old_line diff-line-num old" data-linenumber="1060" style="width: 35px; color: rgba(31,30,36,.24); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: inherit;" align="right" bgcolor="#f9d7dc">
1060
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<td class="new_line diff-line-num old" data-linenumber="1060" style="width: 35px; color: rgba(31,30,36,.24); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: inherit;" align="right" bgcolor="#f9d7dc">
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<td class="line_content old" style="padding: inherit;" bgcolor="#fbe9eb"><pre style='display: block; font-size: 14px; color: #333238; position: relative; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'>-<span id="LC1060" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Tricky examples in: #19042, #17672</span></span>
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<td class="old_line diff-line-num new" data-linenumber="1061" style="width: 35px; color: rgba(31,30,36,.24); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: inherit;" align="right" bgcolor="#ddfbe6">
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<td class="new_line diff-line-num new" data-linenumber="1060" style="width: 35px; color: rgba(31,30,36,.24); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: inherit;" align="right" bgcolor="#ddfbe6">
1060
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<td class="line_content new" style="padding: inherit;" bgcolor="#ecfdf0"><pre style='display: block; font-size: 14px; color: #333238; position: relative; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'>+<span id="LC1060" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Tricky examples in: #19042, #17672, #24984. The last (#24984) is particular subtle:</span></span>
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<td class="new_line diff-line-num new" data-linenumber="1061" style="width: 35px; color: rgba(31,30,36,.24); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: inherit;" align="right" bgcolor="#ddfbe6">
1061
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<td class="line_content new" style="padding: inherit;" bgcolor="#ecfdf0"><pre style='display: block; font-size: 14px; color: #333238; position: relative; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'>+<span id="LC1061" class="line" lang="haskell"></span>
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<td class="old_line diff-line-num new" data-linenumber="1061" style="width: 35px; color: rgba(31,30,36,.24); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: inherit;" align="right" bgcolor="#ddfbe6">
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<td class="new_line diff-line-num new" data-linenumber="1062" style="width: 35px; color: rgba(31,30,36,.24); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: inherit;" align="right" bgcolor="#ddfbe6">
1062
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<td class="line_content new" style="padding: inherit;" bgcolor="#ecfdf0"><pre style='display: block; font-size: 14px; color: #333238; position: relative; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'>+<span id="LC1062" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> Inert: [W] g1: F a0 ~ F a1</span></span>
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<td class="new_line diff-line-num new" data-linenumber="1063" style="width: 35px; color: rgba(31,30,36,.24); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: inherit;" align="right" bgcolor="#ddfbe6">
1063
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<td class="line_content new" style="padding: inherit;" bgcolor="#ecfdf0"><pre style='display: block; font-size: 14px; color: #333238; position: relative; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'>+<span id="LC1063" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> [W] g2: F a2 ~ F a1</span></span>
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<td class="new_line diff-line-num new" data-linenumber="1064" style="width: 35px; color: rgba(31,30,36,.24); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: inherit;" align="right" bgcolor="#ddfbe6">
1064
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<td class="line_content new" style="padding: inherit;" bgcolor="#ecfdf0"><pre style='display: block; font-size: 14px; color: #333238; position: relative; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'>+<span id="LC1064" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> [W] g3: F a3 ~ F a1</span></span>
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<td class="new_line diff-line-num new" data-linenumber="1065" style="width: 35px; color: rgba(31,30,36,.24); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: inherit;" align="right" bgcolor="#ddfbe6">
1065
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<td class="line_content new" style="padding: inherit;" bgcolor="#ecfdf0"><pre style='display: block; font-size: 14px; color: #333238; position: relative; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'>+<span id="LC1065" class="line" lang="haskell"></span>
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<td class="old_line diff-line-num new" data-linenumber="1061" style="width: 35px; color: rgba(31,30,36,.24); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: inherit;" align="right" bgcolor="#ddfbe6">
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<td class="new_line diff-line-num new" data-linenumber="1066" style="width: 35px; color: rgba(31,30,36,.24); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: inherit;" align="right" bgcolor="#ddfbe6">
1066
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<td class="line_content new" style="padding: inherit;" bgcolor="#ecfdf0"><pre style='display: block; font-size: 14px; color: #333238; position: relative; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'>+<span id="LC1066" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Now we add [W] g4: F a1 ~ F a7. Should we kick out g1,g2,g3? No! The</span></span>
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<td class="new_line diff-line-num new" data-linenumber="1067" style="width: 35px; color: rgba(31,30,36,.24); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: inherit;" align="right" bgcolor="#ddfbe6">
1067
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<td class="line_content new" style="padding: inherit;" bgcolor="#ecfdf0"><pre style='display: block; font-size: 14px; color: #333238; position: relative; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'>+<span id="LC1067" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">substitution doesn't need to be idempotent, merely terminating. And in #24984</span></span>
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<td class="new_line diff-line-num new" data-linenumber="1068" style="width: 35px; color: rgba(31,30,36,.24); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: inherit;" align="right" bgcolor="#ddfbe6">
1068
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<td class="line_content new" style="padding: inherit;" bgcolor="#ecfdf0"><pre style='display: block; font-size: 14px; color: #333238; position: relative; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'>+<span id="LC1068" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">it turned out that we kept adding one new constraint and kicking out all the</span></span>
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<td class="old_line diff-line-num new" data-linenumber="1061" style="width: 35px; color: rgba(31,30,36,.24); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: inherit;" align="right" bgcolor="#ddfbe6">
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<td class="new_line diff-line-num new" data-linenumber="1069" style="width: 35px; color: rgba(31,30,36,.24); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: inherit;" align="right" bgcolor="#ddfbe6">
1069
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<td class="line_content new" style="padding: inherit;" bgcolor="#ecfdf0"><pre style='display: block; font-size: 14px; color: #333238; position: relative; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'>+<span id="LC1069" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">previous inert ones (and that rewriting led to exponentially big constraints due</span></span>
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<td class="new_line diff-line-num new" data-linenumber="1070" style="width: 35px; color: rgba(31,30,36,.24); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: inherit;" align="right" bgcolor="#ddfbe6">
1070
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<td class="line_content new" style="padding: inherit;" bgcolor="#ecfdf0"><pre style='display: block; font-size: 14px; color: #333238; position: relative; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'>+<span id="LC1070" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">to lack of contraint sharing.) So we only want to look /under/ type family applications.</span></span>
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1061
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<td class="new_line diff-line-num" data-linenumber="1071" style="width: 35px; color: rgba(31,30,36,.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">
1071
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<td class="line_content" style="padding: inherit;"><pre style='display: block; font-size: 14px; color: #333238; position: relative; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'> <span id="LC1071" class="line" lang="haskell"></span>
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<td class="old_line diff-line-num" data-linenumber="1062" style="width: 35px; color: rgba(31,30,36,.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">
1062
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<td class="new_line diff-line-num" data-linenumber="1072" style="width: 35px; color: rgba(31,30,36,.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">
1072
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<td class="line_content" style="padding: inherit;"><pre style='display: block; font-size: 14px; color: #333238; position: relative; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'> <span id="LC1072" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">The proof is hard. We start by ignoring flavours. Suppose that:</span></span>
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<td class="old_line diff-line-num" data-linenumber="1063" style="width: 35px; color: rgba(31,30,36,.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">
1063
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<td class="new_line diff-line-num" data-linenumber="1073" style="width: 35px; color: rgba(31,30,36,.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">
1073
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<td class="line_content" style="padding: inherit;"><pre style='display: block; font-size: 14px; color: #333238; position: relative; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'> <span id="LC1073" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">* We are adding [lhs_w -fw-> rhs_w] to a well-formed, terminating substitution S.</span></span>
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