The SRFI-10 structure defines an octothorpe reader syntax, comma, as specified
in SRFI 10.  It also defines a procedure, DEFINE-READER-CONSTRUCTOR, for user-
defined reader constructors.

(DEFINE-READER-CONSTRUCTOR <symbol> <procedure>) -> unspecific       ;procedure
  Define SYMBOL to be a reader constructor, calling PROCEDURE at read-time;
  that is, for any instances of #,(SYMBOL <arg> ...), PROCEDURE will be applied
  to the arguments ARG ....

The test/ directory contains a couple of examples.  A structure whose name is
of the form SRFI-10-TEST/<foo> corresponds to test/<foo>.scm.

This implementation of SRFI 10 is not perfect: it will give errors regarding
invalid data in quotations if your reader constructors construct anything that
isn't an S-expression, and all that's only if you quote those; if you don't
quote the things, you'll get even worse errors.

There is one built-in reader constructor:

#,(DEFINE-READER-CONSTRUCTOR <name> <procedure-expression>) ;reader constructor
  Just like the DEFINE-READER-CONSTRUCTOR procedure, except that NAME must be a
  literal symbol, and this operates at read-time.  PROCEDURE-EXPRESSION will be
  evaluated in the what interaction environment is in state during read-time.