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(bison.info)Rpcalc Input

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Explanation of `input'
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   Consider the definition of `input':

     input:    /* empty */
             | input line
     ;

   This definition reads as follows: "A complete input is either an
empty string, or a complete input followed by an input line".  Notice
that "complete input" is defined in terms of itself.  This definition
is said to be "left recursive" since `input' appears always as the
leftmost symbol in the sequence.  Note: Recursive Rules.

   The first alternative is empty because there are no symbols between
the colon and the first `|'; this means that `input' can match an empty
string of input (no tokens).  We write the rules this way because it is
legitimate to type `Ctrl-d' right after you start the calculator.  It's
conventional to put an empty alternative first and write the comment
`/* empty */' in it.

   The second alternate rule (`input line') handles all nontrivial
input.  It means, "After reading any number of lines, read one more
line if possible."  The left recursion makes this rule into a loop.
Since the first alternative matches empty input, the loop can be
executed zero or more times.

   The parser function `yyparse' continues to process input until a
grammatical error is seen or the lexical analyzer says there are no more
input tokens; we will arrange for the latter to happen at end of file.