Why is the distinction between mutable and immutable values important in the implementation of a language with a reference model of variables? Define orthogonality in the context of programming...

Why is the distinction between mutable and immutable values important in the implementation of a language with a reference model of variables? Define orthogonality in the context of programming language design. What is the difference between a statement and an expression? What does it mean for a language to be expression-oriented?