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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 is_missing_variable( value_node: ValueNode, variables: Dict[str, Any] = None ) -> bool: """Check if `value_node` is a variable not defined in the `variables` dict."""

    return isinstance(value_node, VariableNode) and (
        not variables or is_invalid(variables.get(value_node.name.value, INVALID))
    )