def stage_signature(vcs, signature):
evidence_path = _get_staged_history_path(vcs)<EOL>staged = get_staged_signatures(vcs)<EOL>if signature in staged:<EOL><INDENT>raise AlreadyStagedError<EOL><DEDENT>staged.append(signature)<EOL>string = '<STR_LIT:\n>'.join(staged)<EOL>with open(evidence_path, '<STR_LIT:w>') as f:<EOL><INDENT>f.write(string)<EOL><DEDENT>
Add `signature` to the list of staged signatures

    Args:
        vcs (easyci.vcs.base.Vcs)
        signature (basestring)

    Raises:
        AlreadyStagedError