<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def score_statement(self, st, extra_evidence=None):
 """Computes the prior belief probability for an INDRA Statement. The Statement is assumed to be de-duplicated. In other words, the Statement is assumed to have a list of Evidence objects that supports it. The prior probability of the Statement is calculated based on the number of Evidences it has and their sources. Parameters st : indra.statements.Statement An INDRA Statements whose belief scores are to be calculated. extra_evidence : list[indra.statements.Evidence] A list of Evidences that are supporting the Statement (that aren't already included in the Statement's own evidence list. Returns ------- belief_score : float The computed prior probability for the statement """

        if extra_evidence is None:
            extra_evidence = []
        all_evidence = st.evidence + extra_evidence
        return self.score_evidence_list(all_evidence)