code: func catShortcutArg(stmt *syntax.Stmt) *syntax.Word {
  	if stmt.Cmd != nil || stmt.Negated || stmt.Background || stmt.Coprocess {
  		return nil
  	}
  	if len(stmt.Redirs) != 1 {
  		return nil
  	}
  	redir := stmt.Redirs[0]
  	if redir.Op != syntax.RdrIn {
  		return nil
  	}
  	return redir.Word
  }
docs: // catShortcutArg checks if a statement is of the form "$(<file)". The redirect
  // word is returned if there's a match, and nil otherwise.