Generic function
Returns the slant component of the specified text style.
text-style-slant text-style => slant
<text-style>.
one-of(#"roman", #"italic", #"oblique", #f).
duim-dcsduim-dcsReturns the slant component of the specified text style.
text-style-components, page 188
text-style-strikeout?, page 191