See
   http://www.webber-labs.com/wp-content/uploads/2015/08/mpl-02.pdf#page=18


 exp := exp '+' exp
      | exp '*' exp
      | '(' exp ')'
      | 'a'
      | 'b'
      | 'c'



left derivation for "a+b"

exp ==> exp+exp
    ==> a+exp
    ==> a+b

         exp     // parse tree
        / | \
       /  |  \
     exp  +   exp
      |        |
      a        b



left derivations for "a*b+c"

exp ==> exp+exp                    exp      // first parse tree
    ==> exp*exp+exp               / | \     // * has higher precedence
    ==> a*exp+exp                /  |  \
    ==> a*b+exp               exp   +   exp
    ==> a*b+c                / | \       |
                            /  |  \      c
exp ==> exp*exp          exp   *   exp
    ==> a*exp             |         |
    ==> a*exp+exp         a         b
    ==> a*b+exp
    ==> a*b+c                      exp      // second parse tree
                                  / | \     // + has higher precedence
                                 /  |  \
                              exp   *   exp
                               |       / | \
                               a      /  |  \
                                    exp  +   exp
                                     |        |
                                     b        c



left derivation for "(a+(b))"

 exp ==> (exp)
     ==> (exp+exp)
     ==> (a+exp)
     ==> (a+(exp))
     ==> (a+(b))
                         exp     // parse tree
                        / | \
                       /  |  \
                      (  exp  )
                        / | \
                       /  |  \
                    exp   +   exp
                     |       / | \
                     a      /  |  \
                           (  exp  )
                               |
                               b