code: def signature(self) -> str:
      child_sigs = []
      for child in self.children:
          child_sigs.append(child.signature())
      return f"{self.get_symbol().lower()}({','.join(child_sigs)})"
  
file_path: evadb/evadb/expression/aggregation_expression.py
imports: from evadb.expression.abstract_expression import AbstractExpression, ExpressionReturnType, ExpressionType
  from evadb.models.storage.batch import Batch
label_desc: Use image node_id 4 for calling the AggregationExpression obj's underlying member method code with example usage: obj.signature() and returns: self, str, str, str