left       = integral_by "f". ("l","r"). "f" "l"
right      = integral_by "f". ("l","r"). "f" "r"
midpoint   = integral_by "f". ("l","r"). "f" div\2. plus/"l" "r"
trapezium  = integral_by "f". ("l","r"). div\2. plus "f"~~/"l" "r"
simpson    = integral_by "f". ("l","r"). div\6. plus:-0. <"f" "l",times/4. "f" div\2. plus/"l" "r","f" "r">