39     PosPred(Box<Expr>),
41     NegPred(Box<Expr>),
43     Seq(Box<Expr>, Box<Expr>),
45     Choice(Box<Expr>, Box<Expr>),
47     Opt(Box<Expr>),
49     Rep(Box<Expr>),
51     RepOnce(Box<Expr>),
53     RepExact(Box<Expr>, u32),
55     RepMin(Box<Expr>, u32),
57     RepMax(Box<Expr>, u32),
59     RepMinMax(Box<Expr>, u32, u32),
63     Push(Box<Expr>),
82                 // TODO: Use box syntax when it gets stabilized.  in map_top_down()
84                     let mapped = Box::new(map_internal(*expr, f));  in map_top_down()
88                     let mapped = Box::new(map_internal(*expr, f));  in map_top_down()
92                     let mapped_lhs = Box::new(map_internal(*lhs, f));  in map_top_down()
93                     let mapped_rhs = Box::new(map_internal(*rhs, f));  in map_top_down()
97                     let mapped_lhs = Box::new(map_internal(*lhs, f));  in map_top_down()
98                     let mapped_rhs = Box::new(map_internal(*rhs, f));  in map_top_down()
102                     let mapped = Box::new(map_internal(*expr, f));  in map_top_down()
106                     let mapped = Box::new(map_internal(*expr, f));  in map_top_down()
110                     let mapped = Box::new(map_internal(*expr, f));  in map_top_down()
114                     let mapped = Box::new(map_internal(*expr, f));  in map_top_down()
118                     let mapped = Box::new(map_internal(*expr, f));  in map_top_down()
122                     let mapped = Box::new(map_internal(*expr, f));  in map_top_down()
126                     let mapped = Box::new(map_internal(*expr, f));  in map_top_down()
130                     let mapped = Box::new(map_internal(*expr, f));  in map_top_down()
150                     // TODO: Use box syntax when it gets stabilized.  in map_bottom_up()
151                     let mapped = Box::new(map_internal(*expr, f));  in map_bottom_up()
155                     let mapped = Box::new(map_internal(*expr, f));  in map_bottom_up()
159                     let mapped_lhs = Box::new(map_internal(*lhs, f));  in map_bottom_up()
160                     let mapped_rhs = Box::new(map_internal(*rhs, f));  in map_bottom_up()
164                     let mapped_lhs = Box::new(map_internal(*lhs, f));  in map_bottom_up()
165                     let mapped_rhs = Box::new(map_internal(*rhs, f));  in map_bottom_up()
169                     let mapped = Box::new(map_internal(*expr, f));  in map_bottom_up()
173                     let mapped = Box::new(map_internal(*expr, f));  in map_bottom_up()
177                     let mapped = Box::new(map_internal(*expr, f));  in map_bottom_up()
181                     let mapped = Box::new(map_internal(*expr, f));  in map_bottom_up()
185                     let mapped = Box::new(map_internal(*expr, f));  in map_bottom_up()
189                     let mapped = Box::new(map_internal(*expr, f));  in map_bottom_up()
193                     let mapped = Box::new(map_internal(*expr, f));  in map_bottom_up()
197                     let mapped = Box::new(map_internal(*expr, f));  in map_bottom_up()
280             Box::new(Expr::Str(String::from("a"))),  in top_down_iterator()
281             Box::new(Expr::Str(String::from("b"))),  in top_down_iterator()
293             Box::new(Expr::Seq(  in identity()
294                 Box::new(Expr::Ident("a".to_owned())),  in identity()
295                 Box::new(Expr::Str("b".to_owned())),  in identity()
297             Box::new(Expr::PosPred(Box::new(Expr::NegPred(Box::new(Expr::Rep(  in identity()
298                 Box::new(Expr::RepOnce(Box::new(Expr::Opt(Box::new(Expr::Choice(  in identity()
299                     Box::new(Expr::Insens("c".to_owned())),  in identity()
300                     Box::new(Expr::Push(Box::new(Expr::Range(  in identity()