func AddExistingFiles(fset *token.FileSet, files []*token.File)
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 CloneFileSet(fset *token.FileSet) *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 FileSetFor(files ...*token.File) *token.FileSet
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 GetLines(file *token.File) []int
GetLines returns the table of line-start offsets from a token.File.