Unicode code category of Char. This is one of Cc, Cf, Cn,
Co, Cs, Ll, Lm, Lo, Lt, Lu, Mc, Me, Mn, Nd, Nl, No, Pc, Pd,
Pe, Pf, Pi, Po, Ps, Sc, Sk, Sm, So, Zl, Zp, Zs. When
testing, a single letter stands for all its subcategories.
E.g. to test form a letter, you can use
unicode_property(C, category('L'))