This file describe the lexical aspects of lambda Prolog as implemented in
Terzo. Please send questions, comments, or corrections to Philip Wickline
<philipw@saul.cis.upenn.edu>.

Reserved Words:
---------------
The following are the reserved words, and as such, may not be used as
identifiers.
	type	kind	local	module	localkind
	infix	
	&	,	;	=>	:-
	(	)	->	%(	)%
	%	:	


Identifiers:
------------
There are three type of identifiers in lambda Prolog: upper case
identifiers, lower case identifiers, and quoted identifiers. Upper and
lower case identifiers are made from upper and lower case letters, the
numbers 0 through 9, and the symbols:

	!	@	$	^	=	~	`	'
	<	>	*	+	?	|	/	-

In addition, no reserved word may be used as an identifier, and the
following restrictions serve to differentiate upper and lower case
identifiers.

Upper case identifiers - Must begin with an upper case letter or an
under-bar (_). Examples: X _XYZ  Z87232aba'''

Lower case identifiers - must begin with a lower case letter or one of
these symbols: ! @ $ ^ = ~ ` < > * + ? | / -. Examples: x ----> x23'

Quoted identifiers are any string not including the tab (\t), space (\ ),
newline (\n), form-feed (\012), or single quote (') characters which is
enclosed in a pair of single quotes ('). Examples: '&^%$##(' 'word' '()'


Built-in types:
---------------
This implementation has a number of built-in types:
integers, real, strings, characters, and IO streams.

Integers and Reals- We use the lexical definitions given in "The
Definition of Standard ML":

	An integer constant is any non-empty sequence of digits,
	possibly proceeded by a negation symbol. A real constant is an
	integer constant, possibly followed by a point (.) and one or
	more digits, possibly followed by an exponent symbol E and an
	integer constant; at least one of the optional parts must
	occur, hence no integer constant is a real constant. Examples:
	0.7 3.32E5 3E~7.
	
We denote the type of integers with int, and the type of reals with real.

Strings - We use the lexical definition of strings given in "The
Definition of Standard ML", that is:

	A string constant is a sequence, between quotes ("), of zero
	or more printable characters (i.e., numbered 33-126), spaces
	or escape sequences. Each escape sequence starts with the
	character \, and stands for a character sequence. The escape
	sequences are:

	\n	A single character interpreted by the system as end-of-line.
	\t	Tab.
	\^c	The control character c, where c may be any character with
		number 64-95. The number of \^c is 64 less than the number
		of c. 
	\ddd	The single character with number ddd (3 decimal digits
		denoting an integer in the interval [0,255]).
	\"	"
	\\	\
	\f..f\	This sequence is ignored, where f..f stands for a
		sequence of one or more formating characters.

	The formating characters are a subset of the non-printable
	characters including at least space, tab, newline,
	form-feed. This last form allows long strings to be written on
	more than one line, by writing \ at the end of one line and at
	the start of the next.

The type of strings is written string.


Streams - Terzo provides two kinds of streams: in_stream and
out_stream. Neither type of stream has any meaningful printing value, and
with the exception of the predefined std_in, std_out, and std_err (which
correspond to the usual Un*x/C notions of standard IO) a user cannot
"enter" a stream.