WITH construct in record package

[Masinter at PARC-MAXC]

This feature has been in the system for a while, but there were a few minor
problems which prevented me from advertising it. I fixed them a couple of
months ago:

(WITH <record-name> <record-instance> . <forms>)  	[special form]

The WITH construct can be used to talk about the fields of a record as if they
were variables within a lexical scope.  <record-name> is the name of a record,
and <record-instance> is an expression which evaluates to an instance of that
record. The expressions in <forms> are evaluated in a way where references to
variables which are field-names of <record-name> are implemented via
field-fetches and SETQs of those variables are implemented via
field-replacement. 

For example, given 
	(RECORD RECN (FLD1 FLD2))
	(SETQ INST (CREATE RECN FLD1 _ 10 FLD2 _ 20))

Then the construct

(with RECN INST
	(SETQ FLD2 (PLUS FLD1 FLD2]

is equivalent to
 (replace FLD2 of INST with (PLUS (fetch FLD1 of INST) (fetch FLD2 of INST]

Note that the substitution is lexical: this operates by actually doing a substitution
inside <forms>.

Larry