"""Core module. Provides the basic operations needed in sympy.
"""

from sympify import sympify
from cache import cacheit
from basic import Basic, Atom, C
from singleton import S
from expr import Expr, AtomicExpr
from symbol import Symbol, Wild, Dummy, symbols, var
from numbers import Number, Float, Rational, Integer, NumberSymbol,\
        RealNumber, Real, igcd, ilcm, seterr, E, I, nan, oo, pi, zoo
from power import Pow, integer_nthroot
from mul import Mul, prod
from add import Add
from mod import Mod
from relational import ( Rel, Eq, Ne, Lt, Le, Gt, Ge,
    Equality, GreaterThan, LessThan, Unequality, StrictGreaterThan,
    StrictLessThan )
from multidimensional import vectorize
from function import Lambda, WildFunction, Derivative, diff, FunctionClass, \
    Function, Subs, expand, PoleError, count_ops, \
    expand_mul, expand_log, expand_func,\
    expand_trig, expand_complex, expand_multinomial, nfloat, expand_power_base
from sets import (Set, Interval, Union, EmptySet, FiniteSet, ProductSet,
        Intersection)
from evalf import PrecisionExhausted, N
from containers import Tuple, Dict
from exprtools import gcd_terms, factor_terms, factor_nc

# expose singletons
Catalan = S.Catalan
EulerGamma = S.EulerGamma
GoldenRatio = S.GoldenRatio

@@ -147,16 +147,36 @@
  Atom, C
+, preorder_traversal
 %0Afrom si

Add preorder_traversal to __init__.py