Algol 68 - ACM Digital Library

20 van Wijngaarden, et al.{Thus the class of protonotions (b) is a subclass of the class ofhypernotions. Hypernotions are used in metaproduction rules (1.1.3.3), inhyper-rules (1.1.3.4), as paranotions (1.1.4.2) and, in their own right, to"designate" certain classes of protonotions (1.1.4.1) .}{A "paranotion" is a hypernotion to which certain special conventionsand interpretations apply, as detailed in 1.1.4.2.}f) A "symbol" is a protonotion ending with 'symbol'. {Note that theparanotion symbol (9.1.1.h) designates a particular occurrence of such aprotonotion.}{Examples:b) 'variable point"c) 'variable point numeral' (8.1.2.1.b)d) "INTREAL" (1.2.1.C)e) 'reference to INTREAI.'f) 'letter a symbol' .Note that the protonotion "twas brillig and the slithy roves' is neither asymbol nor a notion, in that it does not end with "symbol' and noproduction rule can be derived for it. Likewise, "I.E~IS" and "CARR()I.I."are not metanotions in that no metaproduction rules are given for them.}g) In order to distinguish the various usages in the text of this Reportof the terms defined above, the following conventions are adopted:(i) No distinguishing marks {quotes, apostrophes or hyphens} are used inproduction rules, metaproduction rules or hyper-rules;(ii) Metanotions, and hypernotions which stand for themselves {i.e., whichdo not designate protonotions}, are enclosed in quotes;(iii) Paranotions are not enclosed in anything {but, as an aid to thereader, are provided with hyphens where, otherwise, they would havebeen provided with blanks};(iv) All other hypernotions {including protonotions} not covered above areenclosed in apostrophes {in order to indicate that they designate someprotonotion, as defined in 1.1.4.1.a};(v) Typographical display features, such as blank space, hyphen, andchange to a new line or new page, are of no significance (but see alto9.4.d).{Examples:(i) LEAP :: local ; heap ; primal, is a metaproduction rule:(ii) "INTREAL" is a metanotion and designates nothing but itself:(iii) reference.to-INTREAL-identifier, which is not enclosed in apostrophesbut is provided with hyphens, is a paranotion designating a construct(1.1.4.2.a) ;(iv) 'variable point' is both a hypernotion and a protonotion; regarded asa hypernotion, it designates itself regarded as a protonotion;(v) 'reference to real' means the same as 'referencetoreal'.}1.1.3.2. Production rules and production treesALGOL 68 Revised Report 21a) The {derived} "production rules" {b} of the language are thoseproduction rules which can be derived from the "hyper-rules" {1.1.3.4},together with those specified informally in 8.1.4.1.d and 9.2.1.d.b) A "production rule" consists of the following items, in order:an optional asterisk ;a nonempty protonotion N ;a colon ;a nonempty sequence of "alternatives" separated by semicolons ;a point.It is said to be a production rule "for" {the notion (1.1.3.1.c)} N.{The optional asterisk, if present, signifies that the notion is not used inother production rules, but is provided to facilitate discussion in thesemantics. It also signifies that that notion may be used as an"abstraction" (1.1.4.2.b) of one of its alternatives.}c) An "alternative" is a nonempty sequence of "members" separatedby commas.d) A "member" is either(i) a notion {and may then be said to be productive, or nonterminal},(ii) a symbol {which is terminal},(iii) empty, or(iv) some other protonotion {for which no production rule can be derived},which is then said to be a "blind alley".{For example, the member "reference to real denotation" (derivedfrom the hyper-rule 8.0.l.a) is a blind alley.}{Examples:b) exponent part : times ten to the power choice,power of ten. (8.1.2.1.g) •times ten to the power choice :times ten to the power symbol ;letter e symbol. (8.1.2.1.h)c) times ten to the power choice, power of ten •times ten to the power symbol •letter e symbold) times ten to the power choice •power of ten •times ten to the power symbol *letter e symbol }e) A "construct in the strict language" is any "production tree" {f} thatmay be "produced" from a production rule of the language.f) A "production tree" T for a notion N, which is termed the "original"of T, is "produced" as follows: