Parsing languages

[keunen%milou.nrb.be@Warbucks.AI.SRI.COM (Vincent Keunen)]

A friend of mine (born in Prolog land) asked me if there were any
facilities for parsing built in CL or available in PD.
 
For those of you that do not know prolog, there is a facility to allow
one to define "operators" and their priority quite easily.  The basic
prolog parser is then able to parse sentences like:
 
	tom love mary
	block b is on block a
 
etc...
 
I told him lisp usually uses prefix notation (love mary tom) and uses
parentheses for operator priority resolution.  He replied that, still,
it is more natural the prolog way, which i must admit.
 
What do you guys think?
 
How should a language look like?
What are the problems the parser could have?
Is there one somewhere?
Is modifiyng the lisp reader a good solution?
 
Thanks for your help.
 
Vincent Keunen
NRB