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use crate::sexp::{SExp, SExp::*, SLeaf::*, util::*};
impl SExp {
/// Performs substitution on the s-expression.
///
/// Replaces all free occurrences of variables
/// named `name` with the given `value`
/// in this s-expression.
/// ```rust
/// use melisp::sexp::{SExp::*, SLeaf::*, util::*};
///
/// assert_eq!(
/// scons(Add, scons(var("a"), var("b"))).subst("b", &scons(Sub, scons(2, 1))),
/// scons(Add, scons(var("a"), scons(Sub, scons(2, 1))))
/// );
/// ```
pub fn subst(self, name: &str, value: &SExp) -> SExp {
match self {
SCons(a, b) => scons((*a).subst(name, value), (*b).subst(name, value)),
Atom(Var(x)) if x == name => value.clone(),
t => t,
}
}
}
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