Add some comments.

src/main/java/org/nwapw/abacus/Abacus.java

@@ -146,6 +146,11 @@ public class Abacus {
         return tree.reduce(numberReducer);
     }
 
+    /**
+     * Creates a number from a string.
+     * @param numberString the string to create the number from.
+     * @return the resulting number.
+     */
     public NumberInterface numberFromString(String numberString){
         Class<? extends NumberInterface> toInstantiate =
                 pluginManager.numberFor(configuration.getNumberImplementation());

src/main/java/org/nwapw/abacus/parsing/TreeBuilder.java

@@ -4,16 +4,40 @@ import org.nwapw.abacus.tree.TreeNode;
 
 import java.util.List;
 
+/**
+ * TreeBuilder class used to piece together a Tokenizer and
+ * Parser of the same kind. This is used to essentially avoid
+ * working with any parameters at all, and the generics
+ * in this class are used only to ensure the tokenizer and parser
+ * are of the same type.
+ * @param <T> the type of tokens created by the tokenizer and used by the parser.
+ */
 public class TreeBuilder<T> {
 
+    /**
+     * The tokenizer used to convert a string into tokens.
+     */
     private Tokenizer<T> tokenizer;
+    /**
+     * The parser used to parse a list of tokens into a tree.
+     */
     private Parser<T> parser;
 
+    /**
+     * Create a new Tree Builder with the given tokenizer and parser
+     * @param tokenizer the tokenizer to turn strings into tokens
+     * @param parser the parser to turn tokens into a tree
+     */
     public TreeBuilder(Tokenizer<T> tokenizer, Parser<T> parser){
         this.tokenizer = tokenizer;
         this.parser = parser;
     }
 
+    /**
+     * Parse the given string into a tree.
+     * @param input the string to parse into a tree.
+     * @return the resulting tree.
+     */
     public TreeNode fromString(String input){
         List<T> tokens = tokenizer.tokenizeString(input);
         if(tokens == null) return null;