One trick to have an equation only for the M-1 elements is by using the cardinal of a set (the number of elements): card(m) and the order:

myconstraint(m$(ord(m) ne card(m))..

With respect to your second question: A variable is defined over a set, e.g. X(i), but can’t be defined over a variable itself like you do X(Y).
Correct, but I don’t know if this is what you are looking for, is: