let I be set ; :: thesis: for X, Y, Z being ManySortedSet of I holds (X (/\) Y) (/\) Z = X (/\) (Y (/\) Z)
let X, Y, Z be ManySortedSet of I; :: thesis: (X (/\) Y) (/\) Z = X (/\) (Y (/\) Z)
let i be object ; :: according to PBOOLE:def 10 :: thesis: ( i in I implies ((X (/\) Y) (/\) Z) . i = (X (/\) (Y (/\) Z)) . i )
assume A1: i in I ; :: thesis: ((X (/\) Y) (/\) Z) . i = (X (/\) (Y (/\) Z)) . i
hence ((X (/\) Y) (/\) Z) . i = ((X (/\) Y) . i) /\ (Z . i) by Def5
.= ((X . i) /\ (Y . i)) /\ (Z . i) by A1, Def5
.= (X . i) /\ ((Y . i) /\ (Z . i)) by XBOOLE_1:16
.= (X . i) /\ ((Y (/\) Z) . i) by A1, Def5
.= (X (/\) (Y (/\) Z)) . i by A1, Def5 ;
:: thesis: verum