SIM_instruction_child returns the child n
(counting started from 0) for instruction ii. Child
0 will always be valid if the instruction has a child, i.e. if
child 0 is squashed child 1 will become 0 etc.
SIM_instruction_parent returns the parent instruction.
Index Thrown if ii is illegal.
The instruction id of the requested instruction, or
0 if there is none.