grammar Arithmetic
  rule assign
    s variable s '=' expr {
	def value
	  variable.value = expr.value
	end
      }
    / expr
  end

  rule expr
    s term seq:( s add_op s term )* s {
	def value
	  seq.elements.inject(term.value) { |sum, el|
	      el.add_op.operate(sum, el.term.value)
	    }
	end
      }
  end

  rule term
    factor s mul_op s term {	# Right-associative!
	def value
	  mul_op.operate(factor.value, term.value)
	end
      }
    / factor
  end

  rule factor
    variable
    / number
    / '(' expr ')' {
	def value
	  expr.value
	end
      }
  end

  rule variable
    [a-zA-Z_] [a-zA-Z0-9]* {
	def value
	  @@variables[text_value]
	end
	def value=(v)
	  (@@variables ||= {})[text_value] = v
	end
	def variables
	  @@variables
	end
      }
  end

  rule number
    ([1-9] [0-9]* / '0')	{ def value; text_value.to_i; end }
  end

  rule add_op
    '+'				{ def operate(a, b); a+b; end }
    / '-'			{ def operate(a, b); a-b; end }
  end

  rule mul_op
    '*'				{ def operate(a, b); a*b; end }
    / '/'			{ def operate(a, b); a/b; end }
    / '%'			{ def operate(a, b); a%b; end }
  end

  rule s
    [ \t\n\r]*
  end

end