> Let6>> run expr "let\n    a = 4 * [7 + 1 - 1)\n in a * a" :: IO Expr
> 
> 'Program error: user error ('
> '*** lexical error at (line 2, column 13):'
> '    a = 4 * [7 + 1 - 1)'
> '            ^'
> ' in a * a'
> '<end of input>'
> ')'
> Let6>> run expr "let\n    a = 4 * (7 + 1 - 1\n in a * a" :: IO Expr
> 
> 'Program error: user error ('
> '*** syntax error at (line 3, column 3):'
> ' in a * a'
> '  ^'
> '<end of input>'
> '* expected: + or -, * or /, ))'
> Let6>> run expr "let\n    a = 4 * (7 + 1 - 1)\n a * a" :: IO Expr
> 
> 'Program error: user error ('
> '*** syntax error at (line 3, column 2):'
> ' a * a'
> ' ^'
> '<end of input>'
> '* expected: + or -, * or /, in)'
> Let6>> run expr "\n    a = 4 * (7 + 1 - 1)\n in a * a" :: IO Expr
> 
> 'Program error: user error ('
> '*** syntax error at (line 2, column 7):'
> '    a = 4 * (7 + 1 - 1)'
> '      ^'
> ' in a * a'
> '<end of input>'
> '* expected: + or -, * or /, <end of input>)'
> Let6>> run expr "let\n    a = 4 * (7 + - 1)\n in a * a" :: IO Expr
> 
> 'Program error: user error ('
> '*** syntax error at (line 2, column 18):'
> '    a = 4 * (7 + - 1)'
> '                 ^'
> ' in a * a'
> '<end of input>'
> '* expected: <numeral>, <identifier>, let, ()'
> Let6>> run expr "let\n    a = 4  (7 + 1 - 1)\n in a * a" :: IO Expr
> 
> 'Program error: user error ('
> '*** syntax error at (line 2, column 12):'
> '    a = 4  (7 + 1 - 1)'
> '           ^'
> ' in a * a'
> '<end of input>'
> '* expected: + or -, * or /, in)'