theorem :: ALTCAT_1:5

for A, B being set
for F being ManySortedSet of [:B,A:]
for C being Subset of A
for D being Subset of B
for x, y being set st x in C & y in D holds
F . (y,x) = (F | [:D,C:]) . (y,x) by FUNCT_1:49, ZFMISC_1:87;