self as an indented tree.

If debug is True, also prints debug information for each expression arg.

For example:

>>> print(BooleanAlgebra().parse(
...    u'not a and not b and not (a and ba and c) and c or c').pretty())
OR(
  AND(
    NOT(Symbol('a')),
    NOT(Symbol('b')),
    NOT(
      AND(
        Symbol('a'),
        Symbol('ba'),
        Symbol('c')
      )
    ),
    Symbol('c')
  ),
  Symbol('c')
)
r%