Each region relation corresponds to a relational operator in TC. Each relational operator takes two forms, one unary and the other binary. The unary form, op S, generates the set of regions that bear the relation op to some region matching S. For example, in an HTML document, the constraint expression in Paragraph returns all regions that are inside some paragraph element.
The binary form of a relational operator, R op S, generates all regions matching R that bear the relation op to some region matching S. For example, in HTML, Paragraph contains "Lincoln" returns all paragraph elements that contain the string ``Lincoln.''
For the sake of simplicity, all relational operators have equal precedence and right associativity, so that X in Y in Z is parsed as X in (Y in Z).