gotools/internal/tokeninternal/tokeninternal.go

// AddExistingFiles adds the specified files to the FileSet if they
// are not already present. It panics if any pair of files in the
// resulting FileSet would overlap.
func AddExistingFiles(fset *token.FileSet, files []*token.File) { … }

// FileSetFor returns a new FileSet containing a sequence of new Files with
// the same base, size, and line as the input files, for use in APIs that
// require a FileSet.
//
// Precondition: the input files must be non-overlapping, and sorted in order
// of their Base.
func FileSetFor(files ...*token.File) *token.FileSet { … }

// CloneFileSet creates a new FileSet holding all files in fset. It does not
// create copies of the token.Files in fset: they are added to the resulting
// FileSet unmodified.
func CloneFileSet(fset *token.FileSet) *token.FileSet { … }