+ "Parse an expression involving unary and binary operators.
+
+ Within the parsers for operands and operators, the variable NESTEDP is
+ bound to a generalized boolean which is true if an unmatched open-
+ parenthesis has been seen.
+
+ The OPERAND parser should produce a value; the various operator parsers
+ (BINOP, PREOP, and POSTOP) should produce objects obeying the `operator'
+ protocol. The final output of the `expr' parse is the result of
+ evaluating the parsed expression. (Of course, the definition of
+ `evaluation' here is determined entirely by the methods on
+ `apply-operator', so the final value may be a parse tree, for example.)"
+