theorem :: BORSUK_5:7

for x1, x2, x3, x4, x5, x6, x7 being set st x1,x2,x3,x4,x5,x6 are_mutually_distinct & {x1,x2,x3,x4,x5,x6} misses {x7} holds
x1,x2,x3,x4,x5,x6,x7 are_mutually_distinct