let c be object ; :: thesis: ( c in dom ((r (#) f) | X) implies ((r (#) f) | X) . c = (r (#) (f | X)) . c )
assume A2: c in dom ((r (#) f) | X) ; :: thesis: ((r (#) f) | X) . c = (r (#) (f | X)) . c
then A3: c in (dom (r (#) f)) /\ X by RELAT_1:61;
then A4: c in X by XBOOLE_0:def 4;
A5: c in dom (r (#) f) by A3, XBOOLE_0:def 4;
then c in dom f by VALUED_1:def 5;
then c in (dom f) /\ X by A4, XBOOLE_0:def 4;
then A6: c in dom (f | X) by RELAT_1:61;
then A7: c in dom (r (#) (f | X)) by VALUED_1:def 5;
thus ((r (#) f) | X) . c = (r (#) f) . c by A2, FUNCT_1:47
.= r * (f . c) by A5, VALUED_1:def 5
.= r * ((f | X) . c) by A6, FUNCT_1:47
.= (r (#) (f | X)) . c by A7, VALUED_1:def 5 ; :: thesis: verum