replace expressions with polynomials, add interpreter command class, rewrite parser

lib/ruby_algebra/polynomial.rb

@@ -171,7 +171,9 @@ module RubyAlgebra
     end
 
     def *(other)
-      return Term.new if @coeff.zero? || other.coeff.zero?
+      return Term.new if zero? || other.zero?
+
+      return Term.new(coeff * other, variables) unless other.is_a? Term
 
       new_variables = @variables.clone
       other.variables.each do |symbol, power|
@@ -205,11 +207,19 @@ module RubyAlgebra
       unless @variables.empty?
         first = true
         @variables.each do |var, power|
-          result += if first
-                      "#{var}^#{power}"
-                    else
-                      " * #{var}^#{power}"
-                    end
+          if first
+            unless power == 1
+              result += "#{var}^#{power}"
+            else
+              result += "#{var}"
+            end
+          else
+            unless power == 1
+              result += " * #{var}^#{power}"
+            else
+              result += " * #{var}"
+            end
+          end
           first = false
         end
       end