'which(program) -> str
Minor modification to just directly invoking ``which`` on the remote
system which adds the current working directory to the end of ``$PATH``.'

def which(self, program):

    if (os.path.sep in program):
        return program
    result = self.run(('export    PATH=$PATH:$PWD;    which    %s' % program)).recvall().strip()
    if (('/%s' % program) not in result):
        return None
    return result
