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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def true_num_genes(model, custom_spont_id=None):
 """Return the number of genes in a model ignoring spontaneously labeled genes. Args: model (Model):
 custom_spont_id (str):
 Optional custom spontaneous ID if it does not match the regular expression ``[Ss](_|)0001`` Returns: int: Number of genes excluding spontaneous genes """

    true_num = 0
    for gene in model.genes:
        if not is_spontaneous(gene, custom_id=custom_spont_id):
            true_num += 1
    return true_num