def get_process(pdef, pid):
    """
    Given a process id (that is a string) and a set of process definitions, this fucntion compute
    the process semantics associated with pid.

    :param pid: process identifier
    :param pdef:  process definitions
    :return: process function
    """
    if pid in pdef:
        return pdef[pid]
    else:
        return None