trying to be clever, we reused the memory allocated for the left
operand when doing partial merges
this had been tested to be safe, in general. however, the
implementation was then written such that we always reused
globally defined operands, this meant that we mutated
the operands which were intended to always represent
+1/-1
this then cascades quickly to making increment/decrement
values much larger/smaller than they should be
related to #197
