class Tracer():
    def __init__(self):
        raise Exception("Use BaseTracer instead of Tracer")

    def _push(self, op, obj):
        return CompTracer(self, obj, op)

    def __add__(self, other):
        return self._push("+", other)

    def __sub__(self, other):
        return self._push("-", other)

    def __mul__(self, other):
        return self._push("*", other)

    def __truediv__(self, other):
        return self._push("/", other)

    def __pow__(self, other):
        return self._push("**", other)

    def __pos__(self):
        return self

    def __neg__(self):
        return self._push("*", -1)

    def __repr__(self):
        v = self.eval()
        return "<Tracer: "+str(self)+" = "+str(v if v != None else 'Undefined')+">"

class BaseTracer(Tracer):
    def __init__(self, name, val=None):
        self.name = name
        self.val = val

    def eval(self):
        return self.val

    def __str__(self):
        return self.name

class CompTracer(Tracer):
    def __init__(self, lParent, rParent, op):
        if not isinstance(lParent, Tracer):
            lParent = BaseTracer(repr(lParent), lParent)
        if not isinstance(rParent, Tracer):
            rParent = BaseTracer(repr(rParent), rParent)
        self.parents = [lParent, rParent]
        self.op = op

    def eval(self):
        l = self.parents[0].eval()
        r = self.parents[1].eval()
        if None in [l,r]:
            return None
        if self.op=="+":
            return l + r
        elif self.op=="-":
            return l - r
        elif self.op=="*":
            return l * r
        elif self.op=="/":
            return l / r
        elif self.op=="**":
            return l ** r

    def __str__(self):
        return "("+str(self.parents[0])+" "+self.op+" "+str(self.parents[1])+")"