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Explanation of `input'
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.
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