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The CSL 1.0.1 schema permits multiple values on the position attribute. Reading the specification of the position attribute, I can't see a case in which multiple values would be necessary; and it seems that indeed none of the styles in the repo use multiple values:
~/src/all-styles$ grep 'position=\"[^\"]* [^\"]*\"' *.csl
Would it be possible to amend the schema to allow only one value on this attribute?