I have a problem to write syntax of this simple formula “n not equal m” in equation section. This n, m are sets with n = {1,2,3} and m = {1,2,3,4}. I try to solve this problem by using this syntax to represent that n is not equal to m

notequalm(m) … m - n =g= abs(1);

However, I get these error warnings :

108 notequalm(m) … m - n =g= abs(1);
**** $148 $148,653

148 Dimension different - The symbol is referenced with more/less
indices as declared
257 Solve statement not checked because of previous errors
653 The left side of an equation cannot be a set expression

How to solve this problem? Thank you for the asnwers.