There is a property of languages with respect to copula (a verb 'to be' to mark equivalent thing): the copula may be necessary, or prohibited (and more complex mixtures of necessary and prohibited.
For example, English requires a copula:
He is a doctor (you can't take a way the 'is')
In Russian, 'is' is not allowed (but it is necessary in the past):
On vrach. ('he doctor'..)
Is this property an accepted primary parameter of language (like pro-drop, or head direction) that is, a property that is correlated with other properties? Or is it associated secondarily with one of the more common parameters?