truth of ()

[Jonathan Rees <jar@zohar.ai.mit.edu>]

There was no objection to the proposal that the truth behavior of the
empty list be unspecified the last two times that the issue came up on
this list, so I request that you make this change for R4RS.

[Review of this issue:

1. R3RS specifies that the empty list must be treated as false by
conditionals.

2. The question of ()'s truth was on the agenda for the Snowbird
meeting but we didn't get around to discussing it.

3. Someone requested that we require empty list and false to be
distinct objects.  This request was rejected at Snowbird on
compatibility grounds.

4. Someone requested that the empty list MUST be treated as true.
This is possible only if empty list and false are necessarily
distinct, and this is not the case.]