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The action table (supplied by generated subclass). This table is
indexed by state and terminal number indicating what action is to
be taken when the parser is in the given state (i.e., the given state
is on top of the stack) and the given terminal is next on the input.
States are indexed using the first dimension, however, the entries for
a given state are compacted and stored in adjacent index, value pairs
which are searched for rather than accessed directly (see get_action()).
The actions stored in the table will be either shifts, reduces, or
errors. Shifts are encoded as positive values (one greater than the
state shifted to). Reduces are encoded as negative values (one less
than the production reduced by). Error entries are denoted by zero.