use crate::sexp::SExp;

impl SExp {

    /// Evaluates the s-expression one step.
    ///
    /// **Integer operations**
    ///
    /// Addition, subtraction, multiplication and division
    /// are supported.
    /// ```rust
    /// use melisp::sexp::{SExp::*, SLeaf::*, util::*};
    ///
    /// assert_eq!(
    ///     sexp(Add, sexp(1, 1)).step().unwrap(),
    ///     Atom(Int(2))
    /// );
    ///
    /// assert_eq!(
    ///     sexp(Sub, sexp(1, 2).step().unwrap()),
    ///     Atom(Int(-1))
    /// );
    ///
    /// assert_eq!(
    ///     sexp(Mul, sexp(2, 3).step().unwrap()),
    ///     Atom(Int(6))
    /// );
    ///
    /// assert_eq!(
    ///     sexp(Div, sexp(6, 2).step().unwrap()),
    ///     Atom(Int(3))
    /// );
    /// ```
    ///
    /// Division truncates the decimals.
    /// ```rust
    /// use melisp::sexp::{SExp::*, SLeaf::*, util::*};
    ///
    /// assert_eq!(
    ///     sexp(Div, sexp(5, 2).step().unwrap()),
    ///     Atom(Int(2))
    /// );
    ///
    /// ```
    pub fn step(self) -> Result<Self, String> {
        todo!();
    }
}