Build a program to take a BNF grammar, read the grammar and then
it will allow a person to build a sentence in the language specified
by the grammar.
Assume the BNF is correct.  No BNF rule will have more than 15 items
on the right hand side. 
All item in a rule are separated by white space.
(You can require exactly one space between items).
A rule completes in one line.  
You can limit the number of rules to 50.

The BNF rules (referred to as a grammar) are stored in a file.
Your program is to read and echo the file, and then walk 
the user through the process of building a sentence.

Once the grammar is read, the program will prompt the user to
build a sentence in the specified grammar.  For example assume the
grammar is:

  <s> ->  let <var> = <val>
  <val> -> <var> | <num>
  <var> -> a | b
  <num> -> 1 | 2 | 3

The program and the user could have the following interaction
(user answers after the ?):
deriving <s> 
deriving <var>
specify number of choice: 1 a, 2 b
?2
deriving <val>
specify number of choice: 1 <var>, 2 <num>
?2
deriving <num>
specify number of choice: 1 1, 2 2, 3 3
?1
The derived sentence is:
let b = 1

Assume the left hand side of the first rule specifies the start
symbol.  Demonstrate the program with at least two grammars and for
each grammar at least two examples.

If you do not have test data for a feature, it is assume it
is not in the code and you will be graded accordingly.  It does
not matter if it is in the code or not.