a is expanded at time 1.
contents of a:   a is expanded at time <use time>.
  b is expanded at time 0.
contents of b:   b is expanded at time 0.
  c is expanded at time 1;   a is expanded at time 1.
  d is expanded at time 1;   a is expanded at time 2.
  e is expanded at time 2;   a is defined for the 2nd time <use time>.