ConnectorExpander cannot be applied tier-wise in a model

[qtothec]

Expected behavior: m.b1.link is expanded by the first apply invocation and m.link is expanded by the second apply invocation.
Actual behavior: the second apply invocation tries to re-expand m.b1.link and raises a RuntimeError.

from pyomo.environ import *
from pyomo.core.base.connector import ConnectorExpander

m = ConcreteModel()
m.b1 = Block()
m.b1.x1 = Var()
m.b1.p1 = Connector()
m.b1.p1.add(m.b1.x1, 'X')
m.b1.x2 = Var()
m.b1.p2 = Connector()
m.b1.p2.add(m.b1.x2, 'X')
m.b1.link = Constraint(expr=m.b1.p1 == m.b1.p2)

xfrm = ConnectorExpander()
xfrm.apply(instance=m.b1)

m.x3 = Var()
m.p3 = Connector()
m.p3.add(m.x3, 'X')
m.link = Constraint(expr=m.b1.p1 == m.p3)
xfrm.apply(instance=m)

[blnicho]

Fixed in #583