Maxima Function
elementp (x, a)
Returns true if and only if x is a member of the set a.
true
elementp complains if a is not a literal set.
elementp
Examples:
(%i1) elementp (sin(1), {sin(1), sin(2), sin(3)}); (%o1) true (%i2) elementp (sin(1), {cos(1), cos(2), cos(3)}); (%o2) false