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 Th20:
  G is X_equal-in-line & 1 <= i & i < len G implies v_strip(G,i) =
  { |[r,s]| : G*(i,1)`1 <= r & r <= G*(i+1,1)`1 }
proof
  assume
A1: G is X_equal-in-line;
  0 <> width G by MATRIX_0:def 10;
  then
A2: 1 <= width G by NAT_1:14;
  assume 1 <= i & i < len G;
  hence thesis by A1,A2,GOBOARD5:8;
end;
