11.2 Associations
The transformation process is specified by a collection of associations.
[152] association = (=> query-expression transform-expression priority-expression?)
[153] query-expression = expression
[154] transform-expression = expression
[155] priority-expression = expression
Each association has up to three components:

•  a query-expression returning a node-list; an association is potentially applicable to any node in the node-list returned by its query-expression.

•  a transform-expression that is evaluated for each of the nodes to which the association is applicable.  The value returned describes the node or nodes in the result grove corresponding to the selected node in the source grove.

•  an optional priority-expression that affects whether the association actually applies to a node to which it is potentially applicable.
A query-expression shall evaluate to a node-list.  All the nodes in the node-list returned by a query-expression shall be nodes in the current grove or shall be nodes in an auxiliary grove whose source grove is the current grove.  Auxiliary groves are described in section 9.5, Auxiliary Groves.  In a query-expression, the current-root procedure  and current-node procedure return a singleton node-list containing the root of the current grove.  
A priority-expression shall evaluate to an integer.  The number specifies the priority of the association.  If the priority-expression is omitted for an association, the priority of the association is 0.  Larger numbers indicate higher priorities.
Each node to which an association is potentially applicable has a constituent set of nodes in the current grove.  When the node is in the current grove, the constituent set contains just that node. When the node is in an auxiliary grove, then the constituent set contains the nodes in the current grove that occur in the value of the source property of the node in the auxiliary grove.  An association is actually applicable to any node, n, to which it is potentially applicable unless some higher priority association applies to a node whose constituent set contains a node that is in the constituent set of n.