The mode parameter to the Parse* functions is a set of flags (or 0). They control the amount of source code parsed and other optional parser functionality.
const (
    PackageClauseOnly uint = 1 << iota // parsing stops after package clause
    ImportsOnly                        // parsing stops after import declarations
    ParseComments                      // parse comments and add them to AST
    Trace                              // print a trace of parsed productions
    DeclarationErrors                  // report declaration errors
)