diff --git a/lib/ruby_algebra/parser.rb b/lib/ruby_algebra/parser.rb
index ef9d881..98f2a70 100644
--- a/lib/ruby_algebra/parser.rb
+++ b/lib/ruby_algebra/parser.rb
@@ -57,7 +57,7 @@ module RubyAlgebra
       end
     end
 
-    def tokenize(e)
+    def self.tokenize(e)
       i = 0
       add_implicit_mul = false
       while i < e.length
@@ -123,7 +123,7 @@ module RubyAlgebra
       yield EndToken.new
     end
 
-    def parse(expr)
+    def self.parse(expr)
       operators = []
       out_stack = []
       tokenize(expr) do |token|
@@ -158,7 +158,7 @@ module RubyAlgebra
       out_stack[0]
     end
 
-    def _parse_make_op(operators, out_stack)
+    def self._parse_make_op(operators, out_stack)
       oper = operators.pop
       unless (oper.type == :op && out_stack.length >= 2) || (oper.type == :unary_op && out_stack.length >= 1)
         raise Exception.new
@@ -187,7 +187,7 @@ module RubyAlgebra
       end
     end
 
-    def _parse_op_prio(op_token)
+    def self._parse_op_prio(op_token)
       case op_token.op
       when :add, :sub
         1