COND/DO consistency

["Michael R. Blair" <ziggy@hx.lcs.mit.edu>]

Consider: [paraphrase of 4.2.1]

| (COND (<test> <expression>...)
|       (<test> <expression>...)
|       ...
|       (ELSE <expression> <expression>...))
|
| For the first <test> that evaluates to a true value, the value of
| the last <expression> in that clause is returned as the result of the
| COND. If that clause contains only a <test> and no <expression>s,
| then the value of the <test> is returned as the result.

Compare with: [paraphrase of 4.2.4]

| (DO ((<variable> <init> <step>)
|      ...)
|     (<test> <expression>...)
|   <command>...)
|
| If the <test> evaluates to a true value, then the value of the last
| <expression> is returned as the result of the DO. If no <expression>s
| are present, then the value of the DO expression is unspecified.

I propose that this last line read "then the value of the <test> is
returned as the value of the DO expression." for consistency with the
way test clauses are handled in COND.

Consequently, I propose that the rewrite rule for DO in section 7.3
be changed to:

| (DO ((<variable_1> <init_1> <step_1>)
|      ...)
|     (<test> <expression>...)  ;; NB: <sequence> changed to <expr>...
|   <command_1>...)
|
| = (LETREC ((<loop>
|              (LAMBDA (<variable_1>...)
|                 (COND (<test> <expression>...)        ;; NB: COND
|                       (ELSE <command_1>               ;;  not IF
|                             ...                       ;;
|                             (<loop> <step_1>...)))))) ;;
|       (<loop> <init_1>...))

This is a virtually effortless change which enhances the regularity
of the language.
						~Ziggy