diff --git a/src/main/java/com/github/sommeri/less4j/LessFunction.java b/src/main/java/com/github/sommeri/less4j/LessFunction.java
index 0373dda4..ea2e5a01 100644
--- a/src/main/java/com/github/sommeri/less4j/LessFunction.java
+++ b/src/main/java/com/github/sommeri/less4j/LessFunction.java
@@ -2,13 +2,47 @@
 
 import java.util.List;
 
+import com.github.sommeri.less4j.core.ast.ComposedExpression;
 import com.github.sommeri.less4j.core.ast.Expression;
 import com.github.sommeri.less4j.core.ast.FunctionExpression;
 
+/**
+ * Implement this interface to create custom less function. 
+ *
+ */
 public interface LessFunction {
 
+  /**
+   * Returns true if this function can evaluate the less function.    
+   * 
+   * @param input - function to be evaluated
+   * @param parameters - function parameters 
+   * 
+   * @return <code>true</code> only if the implementation can evaluate the input function.
+   */
   public boolean canEvaluate(FunctionExpression input, List<Expression> parameters);
 
+  /**
+   * Evaluates less function in parameter. Will be called only if {@link #canEvaluate(FunctionExpression, List)} returns <code>true</code>.
+   * 
+   * The <code>evaluatedParameter</code> contains function arguments as parsed into a single abstract 
+   * syntax tree node. A function called with multiple arguments would be sent an instance of 
+   * {@link ComposedExpression} with  arguments bundled in as childs. 
+   * 
+   * The <code>evaluatedParameter</code> contains list of function arguments. It is convenience argument 
+   * and contains <code>evaluatedParameter</code> expression split by commas.
+   * 
+   * @param input - input function
+   * @param parameters - function arguments split (evaluated)
+   * @param evaluatedParameter - all function arguments as a single expression (evaluated)  
+   * @param problems - errors and warnings collector
+   * 
+   * @return result of function evaluation. Must NOT return <code>null</code> and should return 
+   *     correct abstract syntax tree node. Eg. 
+   *     * solved parent child relationships (both ways), 
+   *     * each node instance can appear in the three only once,
+   *     * correctly filled underlying structure properties.
+   */
   public Expression evaluate(FunctionExpression input, List<Expression> parameters, Expression evaluatedParameter, LessProblems problems);
 
 }
diff --git a/src/main/java/com/github/sommeri/less4j/LessProblems.java b/src/main/java/com/github/sommeri/less4j/LessProblems.java
index 36f3e7ad..6ed25578 100644
--- a/src/main/java/com/github/sommeri/less4j/LessProblems.java
+++ b/src/main/java/com/github/sommeri/less4j/LessProblems.java
@@ -2,10 +2,29 @@
 
 import com.github.sommeri.less4j.core.ast.ASTCssNode;
 
+/**
+ * Collects problems and suspicious things encountered by custom code.
+ *
+ */
 public interface LessProblems {
   
+  /**
+   * Report an error. If an error is reported, generated css is considered incorrect and will
+   * not be generated.  
+   * 
+   * @param errorNode - ast node that caused the problem. It is used to generate line number and column 
+   *     number preceding error description. 
+   * @param description - description of encountered problem
+   */
   public void addError(ASTCssNode errorNode, String description);
 
+  /**
+   * Warn user. Warning are available to user, but css is generated as usually.
+   * 
+   * @param weirdNode - ast node that caused the problem. It is used to generate line number and column 
+   *     number preceding error description.
+   * @param description - description of encountered problem
+   */
   public void addWarning(ASTCssNode weirdNode, String description);
   
 }
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