theorem Th72:
  p is x-context_including & x-context_pos_in p <> i in dom p implies
  p/.i is x-omitting
  proof
    assume
A1: p is x-context_including & x-context_pos_in p <> i in dom p;
    then consider j such that
A2: j in dom p & p.j is context of x &
    for n being Nat for t st n in dom p & n <> j & t = p.n
    holds t is x-omitting;
A3: x-context_pos_in p = j by A1,A2,CPI;
    p/.i = p.i by A1,PARTFUN1:def 6;
    hence p/.i is x-omitting by A1,A2,A3;
  end;
