(asdf:component-visited-p operation component)
|Function: Returns the value stored by a call to VISIT-COMPONENT, if that has been called, otherwise NIL. This value stored will be a cons cell, the first element of which is a computed key, so not interesting. The CDR wil be the DATA value stored by VISIT-COMPONENT; recover it as (cdr (component-visited-p op c)). In the current form of ASDF, the DATA value retrieved is effectively a boolean, indicating whether some operations are to be performed in order to do OPERATION X COMPONENT. If the data value is NIL, the combination had been explored, but no operations needed to be performed.