theorem Th18: :: PRALG_3:18

for I being non empty set
for S being non empty non void ManySortedSign
for A being MSAlgebra-Family of I,S
for o being OperSymbol of S
for y being Element of Args (o,(product A)) st the_arity_of o <> {} holds
y in dom (Commute (Frege (A ?. o)))