can't get this generator to yield a math expression.

won't yield

import itertools as it
import math
import sympy as sy

Energy, Frequency, Kinetic_energy, Mass, Momentum, Wavelength, Work_function, Speed_of_light,Force,Acceleration= sy.symbols("Energy Frequency Kinetic_energy Mass Momentum Wavelength Work_function Speed_of_light Force Acceleration")
Symbols_list=[Energy, Frequency, Kinetic_energy, Mass, 
              Momentum, Wavelength, Work_function, Speed_of_light]
Equations= [sy.Eq(Energy,Mass * Speed_of_light**2),sy.Eq(Force,Mass * Acceleration)]

Term_dict={}
for x in Equations:
    Term_dict[x]= x.free_symbols


def associate_2_variables(
    eqn1=sy.Eq(Energy,Mass*Speed_of_light**2),
    eqn2=sy.Eq(Force,Mass*Acceleration),
    term1=Force,
    term2=Energy):
    """associate 2 terms using 2 equations 
       with a common term 
       yielding 1 or more equations with those terms
       """
    if not set([term1,term2])-(Term_dict[eqn1]^Term_dict[eqn2]):#if both terms are in the non-assocated elements/
        common_terms=Term_dict[eqn1]&Term_dict[eqn2]
        if common_terms:
            for term in common_terms:
                new_expr = eqn1.subs(term,sy.solve(eqn2,term)[0])
        if new_expr:
            yield(new_expr)
    
something=associate_2_variables()
something.next() 
#StopIteration Error when I run it as a generator but yields 
# the correct expression when I take it out of the generator