eval env (LetExpr name definition expr) = do
    def <- eval env definition
    computed <- (subs expr name def)
    eval env computed
eval env (LetExpr name definition expr) = do
    def <- eval env definition
    computed <- (subs expr name def)
    eval env computed
eval env (LetExpr name definition expr) = do
    def <- eval env definition
    computed <- (subs expr name def)
    eval env computed