[plt-scheme] symbols redefined in SRFIs

Thanks for the detailed analysis.

> The `map' and `for-each' cases are slightly more tricky. MzScheme's
> `map' reports an error when the argument lists are not all the same
> length, whereas SRFI-1's variants allow it. In those cases, the change
> to MzScheme's versions may be undesirable; I'm not sure.

Matter of taste, and it has been debated in R6RS. I personally like

(let ((delta (circular-list 0 1)))
  (map + delta ls))

as a way to increment every other element of ls.

> not tried to finesse the problems with SRFI-1 specifically. If SRFI-1
> is really special, though, we can consider it.

OK. It's just that the SRFI-1 conflicts seem to come up fairly often,
and people were discussing puting up a prominent notice -- so I
thought a fix might be better than a notice...

-- Dan