A bug in IntMap

[Malcolm Wallace]

> The result of split 5 myMap is
>    ({0:=1,1:=0,2:=0,3:=1,6:=-1}, {7:=0,8:=-2})
> where clearly the mapping for 6 is in the wrong half of the result.

Another bunch of interesting results for split, showing a significant
number of off-by-one bugs:

    split 4 {6:=0,7:=0}	= ({},          {6:=0,7:=0})
    split 5 {6:=0,7:=0}	= ({6:=0},      {7:=0})
    split 6 {6:=0,7:=0}	= ({},          {7:=0})
    split 7 {6:=0,7:=0}	= ({6:=0},      {})
    split 8 {6:=0,7:=0} = ({6:=0},      {7:=0})
    split 9 {6:=0,7:=0} = ({6:=0,7:=0}, {})

Regards,
    Malcolm