reserve i,j,k for Nat,
  r,s,r1,r2,s1,s2,sb,tb for Real,
  x for set,
  GX for non empty TopSpace;
reserve GZ for non empty TopSpace;
reserve f for non constant standard special_circular_sequence,
  G for non empty-yielding Matrix of TOP-REAL 2;

theorem Th18:
  G is X_equal-in-line implies v_strip(G,0) = { |[r,s]| : r <= G*( 1,1)`1 }
proof
  0 <> width G by MATRIX_0:def 10;
  then
A1: 1 <= width G by NAT_1:14;
  assume G is X_equal-in-line;
  hence thesis by A1,GOBOARD5:10;
end;
