If a Doc object had been previously parsed, it was possible for
invalid parses to be added. There were two problems:
1) The parse was only being partially erased
2) The RightArc action was able to create a 1-cycle.
This patch fixes both errors, and avoids resetting the parse if one is
present. In theory this might allow a better parse to be predicted by
running the parser twice.
Closes#1253.
Matthew Honnibal