[plt-scheme] XML schema equivalent but for scheme s-expressions

Hi all, I am working on a project where I want to use s-expressions to
store some complex configuration information and I want an easy way to
validate it. If the configuration was in XML I would simply write an
XML schema and use a validating XML parser to validate it. Is there
anything like this out there for normal scheme s-expressions? Also, is
there an easy way to get line numbers when reading in an external file
of s-expressions? For example, if I determine there is a problem with
some part of the configuration, is there a way  that I can get the
line number to print a message to the user where the problem occurred?
Thanks and sorry if these are easy questions. I am new to lisp/scheme.
-Rob