Script is a sequence of lines:

	send EXPR
	expect EXPR , EXPR
	await EXPR [, EXPR]
	on EXPR
	loop [EXPR]
	break [EXPR]
	continue [EXPR]
	set EXPR = EXPR
	if EXPR
	elif EXPR
	else

Lines that are empty, all whitespace, or whose first non-whitespace
character is ; or #, are comments.

EXPR:
	IDENT
	NUMBER
	STRING
	( EXPR )
	- EXPR
	EXPR + EXPR
	EXPR - EXPR
	EXPR * EXPR
	EXPR / EXPR
	EXPR [ EXPR ]
	EXPR < EXPR
	EXPR > EXPR
	EXPR <= EXPR
	EXPR >= EXPR
	EXPR == EXPR
	EXPR != EXPR
	EXPR ? EXPR : EXPR
	[ ]
	IDENT ( EXPR [, EXPR [,EXPR ...]] )
		where IDENT can be:
			$substr
			$len
IDENT:
	[A-Za-z_$][A-Za-z0-9_$]*
NUMBER:
	[0-9]+
	0[xX][0-9a-fA-F]+
	[0-9]*.[0-9]+
	[0-9]+.[0-9]*
STRING:
	" SCHAR* "
SCHAR:
	[ -ÿ] except " or \
	\" \\ \a \b \f \n \r \t
	\OCTAL
	\xHEX

When the first EXPR to expect or on, or the EXPR to loop, break, or
continue, is a simple IDENT, it is used for its name rather than its
value.  (Put parens around it if you want to use a simple variable for
its value.)

Bodies of on, loop, if/elif/else are indicated by indentation.