theorem Th63:
  for i,j being Integer, a,b being Element of C,I st a = i & b = j
  holds a-b = i-j
  proof
    let i,j be Integer;
    let a,b be Element of C,I;
    assume A1: a = i;
    assume b = j;
    then -b = -j by AOFA_A00:55;
    then a+-b = i+-j by A1,AOFA_A00:55;
    hence a-b = i-j;
  end;
