[cairo] How to change Attribute of specific position

[mhao1999 at live.cn]

I have a string "The" of 3 chars, and insert 3 AttrColor: red, green, blue in sequence.

After insertion, I want to change the second attribute, but I googled a lot and could not find a method to do that,

it seems Pango::AttrList does not support subscript operator, also I tried AttrIter and AttrList.change, but don't know how to get it.
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