code: function getOperationVariables(
    operation: NormalizationOperation,
    variables: Variables,
  ): Variables {
    const operationVariables = {};
    operation.argumentDefinitions.forEach(def => {
      let value = def.defaultValue;
      if (variables[def.name] != null) {
        value = variables[def.name];
      }
      operationVariables[def.name] = value;
    });
    return operationVariables;
  }
docs: Determines the variables that are in scope for a given operation given values
  for some/all of its arguments. Extraneous input variables are filtered from
  the output, and missing variables are set to default values (if given in the
  operation's definition).