Is this fix suitable ?

[ponyatov]

class SimpleVariable:
    def __init__(self): self._value = ''
    def __lshift__(self,val): self._value = val
    def __repr__(self): return '<%s>' % self._value

I implemented << operator for variable class, it maybe makes code looks better:

def personWithSimpleVariable(Person):
    Person << 'Alpha' ; yield Person
    Person << 'Beta'  ; yield Person
    Person << 'Gamma' ; yield Person

I also return binded variable, not just false like it done in original -- it looks more logical.
In result prints looks like:

P = Var() ; print P
for p in Persons(P): print p

<>
<Alpha>
<Beta>
<Gamma>

First print returns unbinded variable, maybe it should looks differ then binded?

[ponyatov]

More logical, shows unbinded w/o <>:

class Var:
    def __init__(self): self._value = None
    def __repr__(self):
        if self._value: return '<%s>' % self._value
        else: return str(self._value)

None
<Alpha>
<Beta>
<Gamma>