set M = { x where x is Element of R : ( x in I & x in J ) } ; { x where x is Element of R : ( x in I & x in J ) } = I /\ J
; then reconsider M = { x where x is Element of R : ( x in I & x in J ) } as Subset of R ;
for y, x being Element of R st x in M holds x * y in M