# XeTeX big and small delimiter

I saw, that in plain.tex file, the delimiters are defines two characters at once. However for Plain XeTeX \XeTeXdelimiter macro description in the documentation only states one character. However I understand, that \delimiter is defining these character for a big and small alternative. Is it possible to define the same with \XeTeXdelimiter?

The reason that I ask this, is that when I write \left|f(x)\right| for instance, then the U+FFFD character appears, instead of something like a big vertical line or bar, when I use Cambria Math. How to define what character to display for \left|?

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