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 {
        todo!();
    }
}