theorem Th59:
  for f st ColVec2Mx f in Solutions_of(A,ColVec2Mx b) holds len f = width A
proof
  let f;
  assume ColVec2Mx f in Solutions_of(A,ColVec2Mx b);
  then
A1: ex g st ColVec2Mx f = ColVec2Mx g & len g = width A by Th58;
  len ColVec2Mx f = len f by MATRIX_0:def 2;
  hence thesis by A1,MATRIX_0:def 2;
end;
