I have a problem with an argument in Fine structure and iteration trees by Mitchell and Steel. Let $E$ be a $(\kappa, \lambda)$-extender. Let $\dot E^{\mathcal{M}}$ the a unary predicate with is interpreted as the extender sequence at $\alpha$. Let $\dot F^{\mathcal{M}}$ be a 3-ary predicate interpreted as the weakly amenable coding of $E_{\alpha}$.
Mitchell and Steel define the ultrapower in the case $\mathcal{M}$ is active. In the first page in http://projecteuclid.org/DPubS/Repository/1.0/Disseminate?handle=euclid.lnl/1235423433&view=body&content-type=pdf_1 there is case 1 where $\mu < \kappa$ ($\mu$ is the critical point of $\dot F^{\mathcal{M}}$).
The authors claim this directly implies that $g$ is constant almost everywhere ($g$ is defined a couple of line before the argument). I don't understand why that is so. Thanks for any help.