geiser-edit-symbol-at-point (M-.) does not work on
fork+exec-command. geiser wrongly tries to lookup fork instead of
looking up fork+exec-command. This is because (thing-at-point 'symbol)
does not correctly identify the whole symbol. And, that, in turn, is
because ?+ has been reclassified as an expression prefix character (')
in .dir-locals.el. It should be a symbol constituent character (_) which
is the default for scheme mode.

(modify-syntax-entry ?+ "'")

This is a rather trivial bug. But, I thought I'll just document it
here. I'm not sure if we should fix it. I can't think of any easy way
out.