code: def asof_join(left, right, predicates=(), by=(), tolerance=None):
      
      return ops.AsOfJoin(left, right, predicates, by, tolerance).to_expr()
docs: Perform an asof join between two tables.  Similar to a left join
      except that the match is done on nearest key rather than equal keys.
  
      Optionally, match keys with 'by' before joining with predicates.
  
      Parameters
      ----------
      left : TableExpr
      right : TableExpr
      predicates : join expression(s)
      by : string
          column to group by before joining
      tolerance : interval
          Amount of time to look behind when joining
  
      Returns
      -------
      joined : TableExpr
          Note that the schema is not materialized yet