let D be non empty set ; :: thesis: for i being natural Number
for T1, T2, T3 being Tuple of i,D
for F being BinOp of D st F is associative holds
F .: ((F .: (T1,T2)),T3) = F .: (T1,(F .: (T2,T3)))
let i be natural Number ; :: thesis: for T1, T2, T3 being Tuple of i,D
for F being BinOp of D st F is associative holds
F .: ((F .: (T1,T2)),T3) = F .: (T1,(F .: (T2,T3)))
let T1, T2, T3 be Tuple of i,D; :: thesis: for F being BinOp of D st F is associative holds
F .: ((F .: (T1,T2)),T3) = F .: (T1,(F .: (T2,T3)))
let F be BinOp of D; :: thesis: ( F is associative implies F .: ((F .: (T1,T2)),T3) = F .: (T1,(F .: (T2,T3))) )
assume A1: F is associative ; :: thesis: F .: ((F .: (T1,T2)),T3) = F .: (T1,(F .: (T2,T3)))

per cases ( i = 0 or i <> 0 ) ;

suppose A2: i = 0 ; :: thesis: F .: ((F .: (T1,T2)),T3) = F .: (T1,(F .: (T2,T3)))

then F .: (T1,T2) = <*> D by Lm1;
then A3: F .: ((F .: (T1,T2)),T3) = <*> D by FINSEQ_2:73;
F .: (T2,T3) = <*> D by A2, Lm1;
hence F .: ((F .: (T1,T2)),T3) = F .: (T1,(F .: (T2,T3))) by A3, FINSEQ_2:73; :: thesis: verum

end;

suppose i <> 0 ; :: thesis: F .: ((F .: (T1,T2)),T3) = F .: (T1,(F .: (T2,T3)))

then reconsider C = Seg i as non empty set ;
A4: T3 is Function of C,D by Lm4;
( T1 is Function of C,D & T2 is Function of C,D ) by Lm4;
hence F .: ((F .: (T1,T2)),T3) = F .: (T1,(F .: (T2,T3))) by A1, A4, FUNCOP_1:61; :: thesis: verum

end;

end;