theorem
  |[x1, y1]| + |[x2, y2]| = |[ x1 + x2, y1 + y2]|
proof
A1: |[x2, y2]|`1 = x2 & |[x2, y2]|`2 = y2 by FINSEQ_1:44;
  |[x1, y1]|`1 = x1 & |[x1, y1]|`2 = y1 by FINSEQ_1:44;
  hence thesis by A1,Th27;
end;
