let X be non empty set ; :: thesis: for Y being set
for F being BinOp of X
for f, g, h being Function of Y,X st F is associative holds
F .: (F .: f,g),h = F .: f,(F .: g,h)
let Y be set ; :: thesis: for F being BinOp of X
for f, g, h being Function of Y,X st F is associative holds
F .: (F .: f,g),h = F .: f,(F .: g,h)
let F be BinOp of X; :: thesis: for f, g, h being Function of Y,X st F is associative holds
F .: (F .: f,g),h = F .: f,(F .: g,h)
let f, g, h be Function of Y,X; :: thesis: ( F is associative implies F .: (F .: f,g),h = F .: f,(F .: g,h) )
assume A1: F is associative ; :: thesis: F .: (F .: f,g),h = F .: f,(F .: g,h)

per cases ( Y = {} or Y <> {} ) ;

suppose Y = {} ; :: thesis: F .: (F .: f,g),h = F .: f,(F .: g,h)

hence F .: (F .: f,g),h = F .: f,(F .: g,h) ; :: thesis: verum

end;

suppose A2: Y <> {} ; :: thesis: F .: (F .: f,g),h = F .: f,(F .: g,h)

now

let y be Element of Y; :: thesis: (F .: (F .: f,g),h) . y = F . (f . y),((F .: g,h) . y)
reconsider x1 = f . y, x2 = g . y, x3 = h . y as Element of X by A2, FUNCT_2:7;
thus (F .: (F .: f,g),h) . y = F . ((F .: f,g) . y),(h . y) by A2, Th48
.= F . (F . (f . y),(g . y)),(h . y) by A2, Th48
.= F . x1,(F . x2,x3) by A1, BINOP_1:def 3
.= F . (f . y),((F .: g,h) . y) by A2, Th48 ; :: thesis: verum

end;

hence F .: (F .: f,g),h = F .: f,(F .: g,h) by A2, Th49; :: thesis: verum

end;

end;