tok_val a
tok_val b
tok_val c

--
1+2 =>
Other

--
1+2*3 =>
Other

--
1*2+3 =>
Other

--
(1+2)*3 =>
Other

--
a+b+c+d =>
Other

--
a*b*3*4 =>
Other

--
1 =>
Const 1

--
a =>
Var a

--
( =>
Parse error: Unexpected token 

--
) =>
Parse error: Expected (

--
(a+b =>
Parse error: Expected )

--
  =>
Parse error: Unexpected token  

--
 $$  =>
Parse error: Unexpected token  
c.i -1
v.name ''
b.op ''
len(c) = 2
s1 = 'bar'
c[1] = 'zz'
s = 'foo'
s = 'zz'
len(c3) = 4
(expr.Binary op:"+" left:(expr.Const i:1) right:(expr.Const i:2))
(expr.Binary
  op: "+"
  left: (expr.Const i:1)
  right: (expr.Binary op:"*" left:(expr.Const i:2) right:(expr.Const i:3))
)
(expr.Binary
  op: "+"
  left: (expr.Binary op:"*" left:(expr.Const i:1) right:(expr.Const i:2))
  right: (expr.Const i:3)
)
(expr.Binary
  op: "*"
  left: (expr.Binary op:"+" left:(expr.Const i:1) right:(expr.Const i:2))
  right: (expr.Const i:3)
)
(expr.Binary
  op: "+"
  left: 
    (expr.Binary
      op: "+"
      left: (expr.Binary op:"+" left:(expr.Var name:a) right:(expr.Var name:b))
      right: (expr.Var name:c)
    )
  right: (expr.Var name:d)
)
(expr.Binary
  op: "*"
  left: 
    (expr.Binary
      op: "*"
      left: (expr.Binary op:"*" left:(expr.Var name:a) right:(expr.Var name:b))
      right: (expr.Const i:3)
    )
  right: (expr.Const i:4)
)
(expr.Const i:1)
(expr.Var name:a)
(expr.Binary op:"+" left:(expr.Const i:-1) right:(expr.Var name:""))