Title: Substitution of expression in list head
Goal: Reject merging of the head of a list comprehension.
Parameters:
    [   {file,"ref_listnot_ok.erl"},
        {posrange,{39,41}},
        {varname,"M"} ].
Documentation:
  The cursor is on "X*2". Neither the head, nor generators of list comprehensions may partake in merging.
