add substitution and differentiation

lib/ruby_algebra/parser.rb

@@ -261,10 +261,17 @@ module RubyAlgebra
                 raise ParserError, "unexpected token #{n.type}, expected ="
               end
               n = tokenizer.next_token
+              unless n.type == :num || n.type == :plus || n.type == :minus
+                raise ParserError, "unexpected token #{n.type}, expected -, + or number"
+              end
+              if n.type == :plus || n.type == :minus
+                negative = true if n.type == :minus
+                n = tokenizer.next_token
+              end
               unless n.type == :num
                 raise ParserError, "unexpected token #{n.type}, expected number"
               end
-              substitutions[variable] = n.value
+              substitutions[variable] = if negative then -n.value else n.value end
             end
             n = tokenizer.next_token
             unless n.type == :paren && n.closing == true