Perplexing type error.

[Michael Jones <Michael.Jones@cl.cam.ac.uk>]

Tres petit(e) abstract en Francais:
Il y a une "type error" que je ne comprende pas.  


While tweaking a preterm parser for type expressions in a proof assistant 
written in caml, I came across the following perplexing type error.  At first, 
it looked like a type inference problem so I specified the types in the 
generic function definitions and got a new error:

#Toplevel input:
>  and parse_pretypepred src = P_APPLY Pred (parse_preterm) src
>                                            ^^^^^^^^^^^^^
This expression has type lexcode list -> preterm * lexcode list,
but is used with type lexcode list -> preterm * lexcode list.
# _

So.  Any ideas why this is an error and how I can fix it?  I'm stumped.  The 
line with the error in it is part of a larger mutualy recursive function.  I 
am using caml-light-0.71 on a sun workstation and didn't see anything similar 
in the known bugs file.  

Some more detail about the line of code:  P_APPLY is a parser combinater that 
takes a function (Pred in this case) and applies it to the parser result.   
Pred is a type constructor for making pretypes out of preterms.  parse_preterm 
is the parser and src is the output of the lex. analyzer.  

regards,
mike.