parameters: (context_set, operator)
docstring: Calculates `+`, `-`, `~` and `not` prefixes.
docstring_summary: Calculates `+`, `-`, `~` and `not` prefixes.
docstring_tokens: Calculates + - ~ and not prefixes .
function: def eval_factor(context_set, operator):
      """
      Calculates `+`, `-`, `~` and `not` prefixes.
      """
      for context in context_set:
          if operator == '-':
              if is_number(context):
                  yield context.negate()
          elif operator == 'not':
              value = context.py__bool__()
              if value is None:  # Uncertainty.
                  return
              yield compiled.create_simple_object(context.evaluator, not value)
          else:
              yield context
function_tokens: def eval_factor ( context_set , operator ) : for context in context_set : if operator == '-' : if is_number ( context ) : yield context . negate ( ) elif operator == 'not' : value = context . py__bool__ ( ) if value is None : # Uncertainty. return yield compiled . create_simple_object ( context . evaluator , not value ) else : yield context