As noted previously , the likelihood of the data are unchanged by adding a constant to the selection coefficient s d in each frequency bin d. We enforce the condition D d=1 s d = 0 to achieve a unique maximum likelihood parameter set . We enforce the constraint by adding the appropriate constant at each iteration . ( Absent any enforcement of the constraint , the MM iterates settle on an arbitrary value ofŝ that indeed satisfies Eq . S13 , but the particular valueŝ depends on the initial condition s ( 0 ) . )