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// Package participle constructs parsers from definitions in struct tags and parses directly into
// those structs. The approach is philosophically similar to how other marshallers work in Go,
// "unmarshalling" an instance of a grammar into a struct.
//
// The supported annotation syntax is:
//
// - `@<expr>` Capture expression into the field.
// - `@@` Recursively capture using the fields own type.
// - `<identifier>` Match named lexer token.
// - `( ... )` Group.
// - `"..."` Match the literal (note that the lexer must emit tokens matching this literal exactly).
// - `"...":<identifier>` Match the literal, specifying the exact lexer token type to match.
// - `<expr> <expr> ...` Match expressions.
// - `<expr> | <expr>` Match one of the alternatives.
//
// The following modifiers can be used after any expression:
//
// - `*` Expression can match zero or more times.
// - `+` Expression must match one or more times.
// - `?` Expression can match zero or once.
// - `!` Require a non-empty match (this is useful with a sequence of optional matches eg. `("a"? "b"? "c"?)!`).
//
// Supported but deprecated:
//
// - `{ ... }` Match 0 or more times (**DEPRECATED** - prefer `( ... )*`).
// - `[ ... ]` Optional (**DEPRECATED** - prefer `( ... )?`).
//
// Here's an example of an EBNF grammar.
//
// type Group struct {
// Expression *Expression `"(" @@ ")"`
// }
//
// type Option struct {
// Expression *Expression `"[" @@ "]"`
// }
//
// type Repetition struct {
// Expression *Expression `"{" @@ "}"`
// }
//
// type Literal struct {
// Start string `@String` // lexer.Lexer token "String"
// End string `("…" @String)?`
// }
//
// type Term struct {
// Name string ` @Ident`
// Literal *Literal `| @@`
// Group *Group `| @@`
// Option *Option `| @@`
// Repetition *Repetition `| @@`
// }
//
// type Sequence struct {
// Terms []*Term `@@+`
// }
//
// type Expression struct {
// Alternatives []*Sequence `@@ ("|" @@)*`
// }
//
// type Expressions []*Expression
//
// type Production struct {
// Name string `@Ident "="`
// Expressions Expressions `@@+ "."`
// }
//
// type EBNF struct {
// Productions []*Production `@@*`
// }
package participle
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