docs: def to_str(self, separator=''):
          '''Build a string from the source sequence.
  
          The elements of the query result will each coerced to a string and then
          the resulting strings concatenated to return a single string. This
          allows the natural processing of character sequences as strings. An
          optional separator which will be inserted between each item may be
          specified.
  
          Note: this method uses immediate execution.
  
          Args:
              separator: An optional separator which will be coerced to a string
                  and inserted between each source item in the resulting string.
  
          Returns:
              A single string which is the result of stringifying each element
              and concatenating the results into a single string.
  
          Raises:
              TypeError: If any element cannot be coerced to a string.
              TypeError: If the separator cannot be coerced to a string.
              ValueError: If the Queryable is closed.
          '''
          if self.closed():
              raise ValueError("Attempt to call to_str() on a closed Queryable.")
  
          return str(separator).join(self.select(str))