diff --git a/MathPad/MPExpression.m b/MathPad/MPExpression.m
index 6c2d7e8..88176d5 100644
--- a/MathPad/MPExpression.m
+++ b/MathPad/MPExpression.m
@@ -22,7 +22,7 @@
 
 
 
-NSString *const MPIllegalElementException = @"MPIllegalElementException";
+NSString *const MPIllegalElementException = @"Illegal Element Exception";
 NSString *const MPIllegalElementExceptionElementKey = @"MPIllegalElementExceptionElementKey";
 
 
@@ -30,15 +30,87 @@ NSString *const MPIllegalElementExceptionElementKey = @"MPIllegalElementExceptio
     NSMutableArray * _elements;
 }
 
+/*!
+    @method     tokens
+    @brief      Private method. Returns an array containing all tokens from the
+                receiver.
+ 
+    @return     All items in the @c MPTokenReferenceFrame.
+ */
 - (NSArray *)tokens;
 
+
+/*!
+    @method     validateElements:
+    @brief      Private method. Checks whether all objects in the specified array
+                are valid expression elements.
+
+    @discussion If an object is not valid a @c MPIllegalElementException is
+                raised.
+ 
+    @param      elements
+                The array of objects to be validated.
+ */
 - (void)validateElements:(NSArray *)elements;
+
+
+/*!
+    @method     fixElements
+    @brief      Private method. Restores consistency in the receiver after a
+                change was made.
+ */
 - (void)fixElements;
 
+
+/*!
+    @method     _replaceSymbolsInRange:withElements:
+    @brief      Private method. Replaces the symbols in the specified range with
+                the elements from the @c elements array.
+
+    @discussion This is the most primitive mutation method of the @c MPExpression
+                class.
+ 
+    @param      range
+                The range of symbols to be replaced. The range is specified in 
+                the symbol reference frame. If the range exceeds the receiver's
+                bounds a @c NSRangeException is raised.
+ 
+    @param      elements
+                The elements that should replace the symbols in the specified
+                range.
+ */
 - (void)_replaceSymbolsInRange:(NSRange)range
                   withElements:(NSArray *)elements;
+
+
+/*!
+    @method     _splitElementsAtLocation:insertionIndex:
+    @brief      Splits the receiver's elements at the specified @c location.
+
+    @discussion The split location can be inside a string element. In that case
+                the string is replaced with two other smaller strings.
+ 
+                Splitting the elements destroys the receiver's integrity. The
+                receiver of this message must also receive a @c -fixElements
+                message soon afterwards.
+ 
+    @param      location
+                The index where to split the elements. Specified in the symbol
+                reference frame.
+ 
+    @param      insertionIndex
+                Splitting elements may shift the indexes of the following
+                elements. This parameter is set to the index (specified in the
+                element reference frame) that should be used to insert a new 
+                element at the location where the elements were previously
+                splitted.
+ 
+    @return     @c YES if a string element was split into two smaller strings, @c
+                NO if the split @c location corresponds to a location between two
+                elements.
+ */
 - (BOOL)_splitElementsAtLocation:(NSUInteger)location
-                  insertionIndex:(out NSUInteger *)insertionIndex;
+                  insertionIndex:(NSUInteger *)insertionIndex;
 
 @end
 
@@ -301,7 +373,7 @@ NSString *const MPIllegalElementExceptionElementKey = @"MPIllegalElementExceptio
 - (NSUInteger)convertIndex:(NSUInteger)anIndex
         fromReferenceFrame:(MPReferenceFrame)fromReferenceFrame
           toReferenceFrame:(MPReferenceFrame)toReferenceFrame
-                    offset:(out NSUInteger *)offset
+                    offset:(NSUInteger *)offset
 {
     if (fromReferenceFrame == toReferenceFrame || anIndex == 0) {
         if (offset) {
@@ -328,7 +400,6 @@ NSString *const MPIllegalElementExceptionElementKey = @"MPIllegalElementExceptio
                 symbolIndex += obj.range.length;
                 *stop = idx >= anIndex - 1;
             }];
-//            symbolIndex = [self.tokens[anIndex] range].location;
             break;
     }
     
@@ -362,24 +433,23 @@ NSString *const MPIllegalElementExceptionElementKey = @"MPIllegalElementExceptio
             
         case MPTokenReferenceFrame:
         {
+            NSUInteger totalLength = 0;
             NSUInteger tokenIndex = 0;
-            while (true) {
-                id<MPToken> token = self.tokens[tokenIndex++];
-                if (NSMaxRange(token.range) < symbolIndex || token.range.location > symbolIndex) {
-                    continue;
-                }
-                NSUInteger offsetInToken = anIndex - token.range.location;
-                if (offsetInToken == token.range.length) {
-                    offsetInToken = 0;
-                    tokenIndex++;
-                }
-                if (offset) {
-                    *offset = offsetInToken;
-                }
-                return tokenIndex;
+            id<MPToken> token;
+            while (totalLength < symbolIndex) {
+                token = self.tokens[tokenIndex++];
+                totalLength += token.range.length;
             }
-            // Should never get here
-            return 0;
+            --tokenIndex;
+            NSUInteger offsetInToken = token.range.length - totalLength + symbolIndex;
+            if (offsetInToken == token.range.length) {
+                offsetInToken = 0;
+                tokenIndex++;
+            }
+            if (offset) {
+                *offset = offsetInToken;
+            }
+            return tokenIndex;
         }
     }
 }
@@ -497,7 +567,7 @@ NSString *const MPIllegalElementExceptionElementKey = @"MPIllegalElementExceptio
 
 
 - (BOOL)_splitElementsAtLocation:(NSUInteger)location
-                  insertionIndex:(out NSUInteger *)insertionIndex
+                  insertionIndex:(NSUInteger *)insertionIndex
 {
     if (location == 0) {
         *insertionIndex = 0;
@@ -562,7 +632,7 @@ NSString *const MPIllegalElementExceptionElementKey = @"MPIllegalElementExceptio
     MPParsedExpression *parsedExpression = [self parse:&parsingErrors];
     NSError *evaluationError;
     NSDecimalNumber *result = parsedExpression ? [parsedExpression evaluate:&evaluationError] : nil;
-    if (errors) {
+    if (errors && (parsedExpression || evaluationError)) {
         NSMutableArray *localErrors = [[NSMutableArray alloc] initWithCapacity:parsingErrors.count+1];
         if (parsingErrors) {
             [localErrors addObjectsFromArray:parsingErrors];