SCANNER

The only unusual aspect of the scanner is that the syntax of string, quoted
atom and character tokens depends on the Prolog flag character_escapes.  In
an attempt to handle this in lex rather than C, the introductor characters
("\"" for strings, "\'" for quoted atoms and "0\'" for characters chooses a
exclusive start-state which depends on the aforementioned Prolog flag.  This
results in 6 start-states (3 constructs x 2 states of the Prolog flag).  In
addition there is a exclusive start-state for COMMENTs.  Also there is a
initial pattern for characters which only recognizes 0', it was necessary to
add a start-state for based integers.  If this was not done and we had two
rules like:

0\'			{ BEGIN ((G.doEsc) ? NUM_ESC_ON : NUM_ESC_OFF); }
{natNum}\'{alphaNum}+	{ V.NAT_S= basedInt(yytext); }

(where basedInt() extracts the based integer from yytext), then a pattern
like 0'0 would match the second pattern (via the longest-match rule) which
is not what is desired.

Characters within strings are returned as STR_CHAR_TOKs.  It is the
parser's responsibility to recognize a sequence of these STR_CHAR_TOKs as a
Prolog list.

Negative numbers are not recognized in the scanner.  Instead, a minus which
immediately (without any intervening whitespace) precedes a digit is
returned as NUM_MINUS_TOK.  This distinguishing by the scanner, allows the
parser to treat the - in the term -1+a as a number sign, while treating the
- in a-1 as a binary operator.  Note that in - 1+a, the - will be treated as
a unary operator.

PARSER

The grammar is essentially a straight-forward transcription of the DEC-10
Quintus grammar.  The major difference is the replacement of the subTerm(M)
non-terminal with an inherited attribute for Term which constrains the
maximum precedence of the Term.

Reduction decisions are made using the %test REDUCE_TEST(n) which looks at
the current lookahead.  It returns FALSE iff the lookahead is not a infix or
suffix operator with precedence < n. 

Operators are recognized using the respective productions at the end of the
grammar.  These use a test which OP(id, type, max) which succeeds if id is
an operator of type and precedence <= max.  It is this test which allows
operator definitions to be changed dynamically.

The non-terminal Term999 is introduced to avoid an attribute-conflict which
would result if it was not used.  The XFX_OP, XFY_OP, etc, non-terminals
were needed for the same reason.

Since comma is overused in the Prolog syntax it needs some special handling.
It is allowed as punctuation to separate structure arguments as well as list
arguments.  It use as an operator is allowed by an alternative for
InfixTerm.  Note that the test REDUCE_TEST() checks for the comma being a
lookahead and if so, treats it like any other infix or suffix operator.

Another slightly problematic feature is the '-' sign.  As indicated above,
the scanner returns a different token NUM_MINUS_TOK if a '-' occurs in the
context where it is immediately followed by a digit.  If when the parser can
accept a unary operator it sees such a NUM_MINUS_TOK followed by a number,
then it absorbs the NUM_MINUS token into the value of the number producing a
negative number.  In other contexts, where the parser is expecting a infix
or suffix operator, a NUM_MINUS_TOK is treated like any other operator
governed by the current declarations.