Why is basic arithmetic disallowed on symbolic functions?

edit

I just found a way to deal with this, which would be to define a python function that creates a new symbolic function out of an already defined variable, while keeping the latex representation of the original variable for illustration purposes:

def f(var1, var2):
    '''
    Creates a symbolic function out of existing variable,
    where the name of the function is f-underscore followed
    by the name of the existing variable, while the latexname
    of the original variable, if defined, stays the same. 
    Returns the function evaluated at another defined variable
    given as var2.
    Example:
    sage: var('B E H T_s')
    sage: eq_B = B == H/E
    sage: eq1 = diff(eq_B.subs(B = f(B,T_s), H = f(H,T_s), E = f(E,T_s)), T_s)
    sage: latex(eq1)
    D[0]\left(B\right)\left(T_{s}\right) = -\frac{H\left(T_{s}\right)
    D[0]\left(E\right)\left(T_{s}\right)}{E\left(T_{s}\right)^{2}} +
    \frac{D[0]\left(H\right)\left(T_{s}\right)}{E\left(T_{s}\right)}
    '''
    name = 'f_'+str(var1)
    z = function(name,latex_name = latex(var1)) 
    return z(var2)