In this paper, we describe a new algebra for XML retrieval. We first describe how to transform an XPath-like query in our algebra. The latter contains a vague predicate, about, which defines a set of document parts within an XML document that fullfill a query expressed as in “flat” Information Retrieval – a query that contains only constraints on content but not on structure. This predicate is evaluated in a probabilistic way: we thus need a probabilistic interpretation of our algebra. Answers to query needs expressed with vague content and vague structure constraints can then beevaluated.