To-Do
 1. Fractions
+2. Tests
 3. expression as unit's val
+4. Revisar reduce y pasar a map
+5. mk :m recurse down primero
 6. Going multiprocess
 7. Functions! Derivate! Integrate!


t = {:mult, 
      {:elev, 
        {:vari, "x"}, 
        {:numb, 2}}, 
      {:suma, 
        {:mult, 
          {:numb, 0}, 
          {:fcall, "ln", [vari: "x"]}},  
        {:mult, 
          {:numb, 2}, 
          {:divi, 
            {:numb, 1}, 
            {:vari, "x"}}}}}