for S being non empty non void ManySortedSign
for o being OperSymbol of S
for U1, U2, U3 being non-empty MSAlgebra over S
for H1 being ManySortedFunction of U1,U2
for H2 being ManySortedFunction of U2,U3
for x being Element of Args (o,U1) holds (H2 ** H1) # x = H2 # (H1 # x)