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| Allegro CL

| 1 Introduction 1.4 Definitions 1.4.1 Notational Conventions 1.4.1.2 Modified BNF Syntax ### 1.4.1.2.1 Splicing in Modified BNF Syntax The primary extension used is the following: [[O]] An expression of this form appears whenever a list of elements is to be spliced into a larger structure and the elements can appear in any order. The symbol O represents a description of the syntax of some number of syntactic elements to be spliced; that description must be of the form O1 | ... | Ol where each Oi can be of the form S or of the form S* or of the form S1. The expression [[O]] means that a list of the form (Oi1 ... Oij) 1 <= j is spliced into the enclosing expression, such that if n <> m and 1 <= n,m <= j, then either Oin <> Oim or Oin = Oim = Qk, where for some 1 <= k <= n, Ok is of the form Qk*. Furthermore, for each Oin that is of the form Qk, that element is required to appear somewhere in the list to be spliced. For example, the expression `(x [[A | B* | C]] y)` means that at most one `A`, any number of `B`'s, and at most one `C` can occur in any order. It is a description of any of these:
     (x y)
     (x B A C y)
     (x A B B B B B C y)
     (x C B A B B B y)
but not any of these:
     (x B B A A C C y)
     (x C B C y)
In the first case, both `A` and `C` appear too often, and in the second case `C` appears too often. The notation [[O1 | O2 | ... ]]+ adds the additional restriction that at least one item from among the possible choices must be used. For example: `(x [[A | B* | C]]+ y)` means that at most one `A`, any number of `B`'s, and at most one `C` can occur in any order, but that in any case at least one of these options must be selected. It is a description of any of these:
     (x B y)
     (x B A C y)
     (x A B B B B B C y)
     (x C B A B B B y)
but not any of these:
     (x y)
     (x B B A A C C y)
     (x C B C y)
In the first case, no item was used; in the second case, both `A` and `C` appear too often; and in the third case `C` appears too often. Also, the expression: `(x [[A | B | C]] y)` can generate exactly these and no others:
     (x A B C y)
     (x A C B y)
     (x A B y)
     (x B A C y)
     (x B C A y)
     (x B A y)
     (x C A B y)
     (x C B A y)

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