[XeTeX] Jump symbol in latex

[Arthur Reutenauer]

> Maybe you could directly input characters U+301A and U+301B, which  
> seem to be the ones you want.

  No, the correct Unicode characters are U+27E6 MATHEMATICAL LEFT WHITE
SQUARE BRACKET and U+27E7 MATHEMATICAL RIGHT WHITE SQUARE BRACKET; but
except for STIX, I doubt many fonts will have them, and it's probably
better to stick to the traditional way of input characters, using TeX
control sequences and ad hoc fonts.

	Arthur