these fields could be absent.

The final line of (\[Wfull\]) represents the Dirac mass couplings of the left-handed leptons with their right-handed counterparts. Again, the fields $S_6$ and $S_7$, both singlets under $SU(2)_A \times SU(2)_B$, carrying only hypercharge $Y=+1$, may or may not be identified depending on the construction, while $\wh{S}_6$ and $\wh{S}_7$, which may or may not be distinct, transform as $(\mathbf{2},\mathbf{2})$. Some of these fields may be absent. We assume that $S_6$, $S_7$, $\wh{S}_6$, and $\wh{S}_7$ do not acquire vevs. Charged lepton masses are then determined by some combination of the coupling matrices $\lambda_6$, $\lambda_7$, $\wh{\lambda}_6$ and $\wh{\lambda}_7$ (and possibly higher-order terms that connect the two sectors) as well as appropriate choices of Higgs vevs for the neutral components of the four Higgs species.

Making Contact with Experimental Data {#sec:data}
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Having laid out the framework for our string-based model, we now wish to ask how well such a structure can accommodate the measurements of neutrino mixing angles and mass differences that have been made, and what sort of predictions (if any) might this framework make in terms of future experimental observations. We use a convention in which the solar mixing data defines the mass difference between $m_2$ 