for a, b, c, d being Real st a < b & b < c & c < d holds
( (TrapezoidalFS (a,b,c,d)) . a = 0 & (TrapezoidalFS (a,b,c,d)) . b = 1 & (TrapezoidalFS (a,b,c,d)) . c = 1 & (TrapezoidalFS (a,b,c,d)) . d = 0 )