style-to-index in style-list%
Returns the index for a particular style. The index for a style's base
style (and shift style, if it is a join style) is guaranteed to be
lower than the style's own index. (As a result, the root style's
index is always 0.) An style's index can change whenever a new
style is added to the list, or the base style or shift style of
another style is changed.
If the given style is not in this list, #f is returned.
( -> exact non-negative integer or send a-style-list style-to-index style)#f
style : style<%> object