Negate an equation:

[compsystems]

eqs:={"x+y<=3","y<=9","c=-x-y"};
expr("neg("+eqs[3]+")"); returns -( c == (-x-y) ) this is a bug?

I do not know why change = to ==, Alters the expression, and becomes a comparison, affecting the logic of an algorithm,
simplify(Ans); returns 0 (false)

should be
eqs:={"x+y<=3","y<=9","c=-x-y"};
expr("neg("+eqs[3]+")"); returns -( c = (-x-y) )
simplify(Ans);
-c = -(-x-y)
-c = (x+y)