Flow object tree construction

• Specified by set of query-construct-specifications.
• This describes a function that can be applied to a node to yield a flow object sequence. Call this the query-construct function.
• The flow object tree is the result of applying this function to the root node.
• A query-construct specification contains a query, a constructor and a priority.
• When the query-construct function is applied to a node it finds the highest priority query-construct specification whose query matches the node and returns the result of applying the constructor function to the node. In other words it's just a big case expression.