theorem Th19:
  Width <*M*> = <*width M*>
proof
  set F=<*M*>;
A1: len F=1 by FINSEQ_1:40;
A3: len F=len (Width F) by CARD_1:def 7;
A4: dom (Width F)=Seg len F by FINSEQ_2:124;
  1 in Seg 1;
  then (Width F).1=width (F.1) by A1,A4,Def4;
  hence thesis by A1,A3,FINSEQ_1:40;
end;
