The present invention provides compounds of formula I, pharmaceutically acceptable salts thereof, and pharmaceutical compositions thereof. The secondary winding thereof may comprise a first tap associated with the first output voltage and a second tap associated with the second output voltage, the first and second taps being configured to be selectively coupled to and uncoupled from an output of the power adapter. The sleeve is pivotally engaged with the housing-such that, while remaining in relatively constant contact with the walls of the cavity during any rotation to change its position, it may occupy substantially any position required along a limited length of the perpendicular adjustment direction.