<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def all_agents(stmts):
 """Return a list of all of the agents from a list of statements. Only agents that are not None and have a TEXT entry are returned. Parameters stmts : list of :py:class:`indra.statements.Statement` Returns ------- agents : list of :py:class:`indra.statements.Agent` List of agents that appear in the input list of indra statements. """

    agents = []
    for stmt in stmts:
        for agent in stmt.agent_list():
            # Agents don't always have a TEXT db_refs entry (for instance
            # in the case of Statements from databases) so we check for this.
            if agent is not None and agent.db_refs.get('TEXT') is not None:
                agents.append(agent)
    return agents