Structured value notation
.........................

Structured value syntax allows a piece of structure to be written where an
identifier would normally be written identifier.  The structure is delimited
by ().

(sp write-top-ten-list*apply-and-reconsider-operator*do-number-seven
  (goal <g> ^problem-space (^name write-top-ten-list)
      ^state <s>
      ^operator (<o> ^name write-item ^number 7 -^exceptional-case)
 -->
  (<s> ^item-seven lloyd-bridges !)
  (<g> ^operator <o> @))

expands into

(sp write-top-ten-list*apply-and-reconsider-operator*do-number-seven
  (goal <g> ^problem-space <p*1> ^state <s> ^operator <o>)
  (<p*1> ^name write-top-ten-list)
  (<o> ^name write-item ^number 7 -^exceptional-case)
 -->
  (<s> ^item-seven lloyd-bridges !)
  (<g> ^operator <o> @))

Attribute path and structured value syntax are orthogonal and can be combined
in any way---a structured value can contain an attribute path, or a structure
can be given as the value for an attribute path.  Structured values may be
nested to any depth.