=== DFS of G1 starting at A ===
  t=1   discover A
  t=2   discover B
  t=3   discover D
  t=4   discover G
  t=5   discover E
           EB finds B gray: back edge
  t=6   discover C
           CA finds A gray: back edge
  t=7   discover F
  t=8   discover H
           HE finds E gray: back edge
  t=9   discover I
           IG finds G gray: back edge
  t=10  finish   I
  t=11  finish   H
  t=12  finish   F
  t=13  finish   C
  t=14  finish   E
  t=15  finish   G
  t=16  finish   D
  t=17  finish   B
  t=18  finish   A
  v          A     B     C     D     E     F     G     H     I
  d/f     1/18  2/17  6/13  3/16  5/14  7/12  4/15  8/11  9/10
  pi         -     A     E     B     G     C     D     F     H
  tree edges (8): AB BD DG GE EC CF FH HI
  back edges (4): EB CA HE IG
  parentheses: (A (B (D (G (E (C (F (H (I I) H) F) C) E) G) D) B) A) 

=== DFS of G1 starting at E ===
  t=1   discover E
  t=2   discover B
  t=3   discover A
  t=4   discover C
           CE finds E gray: back edge
  t=5   discover F
  t=6   discover H
           HE finds E gray: back edge
  t=7   discover I
  t=8   discover G
  t=9   discover D
           DB finds B gray: back edge
  t=10  finish   D
           GE finds E gray: back edge
  t=11  finish   G
  t=12  finish   I
  t=13  finish   H
  t=14  finish   F
  t=15  finish   C
  t=16  finish   A
  t=17  finish   B
  t=18  finish   E
  v          A     B     C     D     E     F     G     H     I
  d/f     3/16  2/17  4/15  9/10  1/18  5/14  8/11  6/13  7/12
  pi         B     E     A     G     -     C     I     F     H
  tree edges (8): EB BA AC CF FH HI IG GD
  back edges (4): CE HE DB GE
  parentheses: (E (B (A (C (F (H (I (G (D D) G) I) H) F) C) A) B) E) 
