theorem
  p is x-context_including implies p<-(x-context_in p) = x-context_pos_in p
  proof
    assume
A1: p is x-context_including;
    then p just_once_values x-context_in p by Th73;
    then p<-(x-context_in p) in dom p &
    p.(p<-(x-context_in p)) = x-context_in p by FINSEQ_4:def 3;
    hence thesis by A1,CPI;
  end;
