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EBNF

当前语法浓缩（用于对照，不是 parser 逐字表）：

File         = "@" PackagePath { CompileTag } { Import } { Decl } .
Import       = [ Ident ] "#" Path  .
ReturnStmt   = "<<" [ Expr ] .
ThrowStmt    = ">>" Expr .
ExitStmt     = "<<<" Expr .
BreakStmt    = "<-" .
ContinueStmt = "->" .
RangeOp      = ":" . // [start, end)
XuStmt       = "xu" QualifiedName "(" [ Args ] ")" .
Expr         = … | Expr "or" "(" Ident ")" Block | Expr "or" ">>" .

PackagePath 使用斜杠路径，域名身份可用引号包围；域名中的点保留。导入别名写在 # 前，外部路径必须有别名。