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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 convert_namespaces_ast( ast, api_url: str = None, namespace_targets: Mapping[str, List[str]] = None, canonicalize: bool = False, decanonicalize: bool = False, ):
 """Recursively convert namespaces of BEL Entities in BEL AST using API endpoint Canonicalization and decanonicalization is determined by endpoint used and namespace_targets. Args: ast (BEL):
 BEL AST api_url (str):
 endpoint url with a placeholder for the term_id (either /terms/<term_id>/canonicalized or /terms/<term_id>/decanonicalized) namespace_targets (Mapping[str, List[str]]):
 (de)canonical targets for converting BEL Entities Returns: BEL: BEL AST """


    if isinstance(ast, NSArg):
        given_term_id = "{}:{}".format(ast.namespace, ast.value)

        # Get normalized term if necessary
        if (canonicalize and not ast.canonical) or (
            decanonicalize and not ast.decanonical
        ):
            normalized_term = convert_nsarg(
                given_term_id,
                api_url=api_url,
                namespace_targets=namespace_targets,
                canonicalize=canonicalize,
                decanonicalize=decanonicalize,
            )
            if canonicalize:
                ast.canonical = normalized_term
            elif decanonicalize:
                ast.decanonical = normalized_term

        # Update normalized term
        if canonicalize:
            ns, value = ast.canonical.split(":")
            ast.change_nsvalue(ns, value)
        elif decanonicalize:
            ns, value = ast.canonical.split(":")
            ast.change_nsvalue(ns, value)

    # Recursively process every NSArg by processing BELAst and Functions
    if hasattr(ast, "args"):
        for arg in ast.args:
            convert_namespaces_ast(
                arg,
                api_url=api_url,
                namespace_targets=namespace_targets,
                canonicalize=canonicalize,
                decanonicalize=decanonicalize,
            )

    return ast