Extend the LET language so that a let declaration can declare an arbitrary number of variables, using the grammar

Expression ::= let {Identifier = Expression} in Expression

As in Schemes let, each of the right-hand sides is evaluated in the current environment, and the body is evaluated with each new variable bound to the value of its associated right-hand side. For example,

let x = 30

in let x = - (x , 1)

y = - (x , 2)

in - (x , y)

should evaluate to 1.