Descriptive Problem Summary:
Byond reference (https://www.byond.com/docs/ref/#/mutable_appearance/var) claims that mutable_appearance has the vis_flags variable however when trying to access it say it is undefined.

For example:

var/mutable_appearance/MA = new()
MA.vis_flags = VIS_HIDE

Will result in:
error: MA.vis_flags: undefined var