code: func (pkg *PkgState) usedCFn() map[string]int {
  	retVal := make(map[string]int)
  	visitor := &usedCFnVisit{retVal}
  	for _, f := range pkg.Files {
  		for _, decl := range f.Decls {
  			fn, ok := decl.(*ast.FuncDecl)
  			if !ok {
  				continue
  			}
  			ast.Walk(visitor, fn)
  		}
  	}
  	return retVal
  }
docs: // useCFn returns the count of how many times a C function has been used in the generated package