theorem Th47:
  len (x1,x2)][(y1,y2)=2 & width (x1,x2)][(y1,y2)=2 & Indices (x1,
  x2)][(y1,y2)=[:Seg 2,Seg 2:]
proof
  set M = (x1,x2)][(y1,y2);
  thus
A1: len M=2 by FINSEQ_1:44;
  rng M = { <*x1,x2*>,<*y1,y2*>} by FINSEQ_2:127;
  then
A2: <*x1,x2*> in rng M by TARSKI:def 2;
  len <*x1,x2*>=2 by FINSEQ_1:44;
  hence width M=2 by A1,A2,Def3;
  hence thesis by A1,FINSEQ_1:def 3;
end;
