> Let4>> run expr "4 * (7 + 1) - 1" :: IO Expr
> Bin (Bin (Const 4) Times (Bin (Const 7) Plus (Const 1))) Minus (Const 1)
> Let4>> run expr "let\n    a = 4 * (7 + 1) - 1\n in a * a" :: IO Expr
> Let ("a" :=: Bin (Bin (Const 4) Times (Bin (Const 7) Plus (Const 1))) Minus (Const 1)) (Bin (Var "a") Times (Var "a"))
> Let4>> run expr "let\n    a = 4 * (7 + 1 - 1\n in a * a" :: IO Expr
> 
> 'Program error: user error ('
> '*** syntax error:'
> ' a * a'
> '<end of input>'
> ')'