code: def _validate_specification(self):
      super(_AsOfMerge, self)._validate_specification()
  
      # we only allow on to be a single item for on
      if len(self.left_on) != 1 and not self.left_index:
          raise MergeError("can only asof on a key for left")
  
      if len(self.right_on) != 1 and not self.right_index:
          raise MergeError("can only asof on a key for right")
  
      if self.left_index and isinstance(self.left.index, MultiIndex):
          raise MergeError("left can only have one index")
  
      if self.right_index and isinstance(self.right.index, MultiIndex):
          raise MergeError("right can only have one index")
  
      # set 'by' columns
      if self.by is not None:
          if self.left_by is not None or self.right_by is not None:
              raise MergeError(
                  "Can only pass by OR left_by " "and right_by"
              )
          self.left_by = self.right_by = self.by
      if self.left_by is None and self.right_by is not None:
          raise MergeError("missing left_by")
      if self.left_by is not None and self.right_by is None:
          raise MergeError("missing right_by")
  
      # add 'by' to our key-list so we can have it in the
      # output as a key
      if self.left_by is not None:
          if not is_list_like(self.left_by):
              self.left_by = [self.left_by]
          if not is_list_like(self.right_by):
              self.right_by = [self.right_by]
  
          if len(self.left_by) != len(self.right_by):
              raise MergeError(
                  "left_by and right_by must be same length"
              )
  
          self.left_on = self.left_by + list(self.left_on)
          self.right_on = self.right_by + list(self.right_on)
  
      # check 'direction' is valid
      if self.direction not in ["backward", "forward", "nearest"]:
          raise MergeError(
              "direction invalid: {direction}".format(
                  direction=self.direction
              )
          )
  
imports: import copy
  import string
  import warnings
  import numpy
  from pandas._libs import hashtable, join, lib
  import pandas.compat
  from pandas.compat import filter, lzip, map, range, zip
  from pandas.errors import MergeError
  from pandas.util._decorators import Appender, Substitution
  from pandas.core.dtypes.common import ensure_float64, ensure_int64, ensure_object, is_array_like, is_bool, is_bool_dtype, is_categorical_dtype, is_datetime64_dtype, is_datetime64tz_dtype, is_datetimelike, is_dtype_equal, is_extension_array_dtype, is_float_dtype, is_int64_dtype, is_integer, is_integer_dtype, is_list_like, is_number, is_numeric_dtype, is_object_dtype, needs_i8_conversion
  from pandas.core.dtypes.missing import isnull, na_value_for_dtype
  from pandas import Categorical, DataFrame, Index, MultiIndex, Series, Timedelta
  import pandas.core.algorithms
  from pandas.core.arrays.categorical import _recode_for_categories
  import pandas.core.common
  from pandas.core.frame import _merge_doc
  from pandas.core.internals import concatenate_block_managers, items_overlap_with_suffix
  import pandas.core.sorting
  from pandas.core.sorting import is_int64_overflow_possible
label_desc: Use image node_id 2 for calling the _AsOfMerge obj's underlying member method code with example usage: obj._validate_specification() without return types