SC27: Test syntax checking of vskip typesetting directive. 1. Test not exactly three arguments (includes vskip). @t vskip @t vskip one @t vskip one two three four 2. Test third argument not "mm". @t vskip 32 xx 3. Test second argument has bad digit. @t vskip 3f mm 4. Test number has more than three digits @t vskip 4355 mm 5. Test a legal one too. @t vskip 20 mm