docs: def apply_projection(projection, value):
      """Apply projection."""
      if isinstance(value, Sequence):
          # Apply projection to each item in the list.
          return [
              apply_projection(projection, item)
              for item in value
          ]
      elif not isinstance(value, Mapping):
          # Non-dictionary values are simply ignored.
          return value
  
      # Extract projection for current level.
      try:
          current_projection = [p[0] for p in projection]
      except IndexError:
          return value
  
      # Apply projection.
      for name in list(value.keys()):
          if name not in current_projection:
              value.pop(name)
          elif isinstance(value[name], dict):
              # Apply projection recursively.
              value[name] = apply_projection(
                  [p[1:] for p in projection if p[0] == name],
                  value[name]
              )
  
      return value