Arg [1]       : Bio::EnsEMBL::Exon exon
  Example       : if ($exonA->equals($exonB)) { ... }
  Description   : Compares two exons for equality.
                  The test for eqality goes through the following list
                  and terminates at the first true match:

                  1. If Bio::EnsEMBL::Feature::equals() returns false,
                     then the exons are *not* equal.
                  2. If both exons have stable IDs: if these are the
                     same, the exons are equal, otherwise not.
                  3. If the exons have the same start, end, strand, and
                     phase, then they are equal, otherwise not.

  Return type   : Boolean (0, 1)

  Exceptions    : Thrown if a non-transcript is passed as the argument.