在xelatex中启用字符突出的正确方法是什么？

documentclass[12pt]{book}

usepackage{xltxtra}
usepackage{polyglossia}

setmainfont[Mapping=tex-text]{Charis SIL}
setmainlanguage{english}

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XeTeXprotrudechars=2
fontf = "Charis SIL:mapping=tex-text"
setprotcode{f}

begin{document}

In a genetic algorithm, a population of strings (called chromosomes or the
genotype of the genome), which encode candidate solutions (called individuals,
creatures, or phenotypes) to an optimization problem, evolves toward better solutions.
Traditionally, solutions are represented in binary as strings of 0s and 1s, but other
encodings are also possible. The evolution usually starts from a population of randomly
generated individuals and happens in generations. In each generation, the fitness of
every individual in the population is evaluated, multiple individuals are
stochastically  selected from the current population (based on their fitness),
and modified (recombined and possibly randomly mutated) to form a new population.
The new population is then used in the next iteration of the algorithm. Commonly,
the algorithm terminates when either a maximum number of generations has been produced,
or a satisfactory fitness level has been reached for the population. If the algorithm
has terminated due to a maximum number of generations, a satisfactory solution may or
may not have been reached.

end{document}