
theorem
{TrapezoidalFS (a,b,c,d) where a,b,c,d is Real : a < b & b < c & c < d}
 c= Membership_Funcs (REAL)
proof
 let x be object;
 assume x in {TrapezoidalFS (a,b,c,d)
 where a,b,c,d is Real : a < b & b < c & c < d}; then
 ex a,b,c,d be Real st x=TrapezoidalFS (a,b,c,d) & a < b & b < c & c < d;
 hence thesis by Def1;
end;
