Exercise 3. (evaluation of arithmetic expressions with variables) let us consider an arithmetic expression described by a tree constructed from tuples as follows: 1. an integer is described by a tuple int(n), where n is an integer. 2. an addition is described by a tuple add(x y), where both x and y are arithmetic expressions. 3. a multiplication is described by a tuple mul(x y), where both x and y are arithmetic expressions. 4. a variable is described by a tuple var(a), where a is an atom giving the variable name. an environment is a record with a label env and for each variable name there is a feature which corresponds to an integer value. for example, the record env(a: 2 b: 4) says that the variable "a" has value 2, whereas the variable "b" has value 4 (note that the oz features should start with a lowercase letter). for example, add(var(a) mul(int(3) var( is an arithmetic expression containing two variables (namely a and b) and its evaluation returns 14. give a specification and an oz implementation of a function eval that takes as arguments an arithmetic expression and an environment, and returns its value. for example, the call {eval add(var(a) mul(int(3) var( env(a: 2 b: 4)} should return 14.