Added Lots of Documentation

Added some nice to haves
Improved and Unified General Code Layout

MathKit/MPElementaryFunctionTerm.m

@@ -9,13 +9,10 @@
 #import "MPElementaryFunctionTerm.h"
 
 #import "MPParsedExpression.h"
-#import "MPPowerFunction.h"
-#import "MPProductTerm.h"
-#import "MPToken.h"
-
-#import "MPExpression.h"
 #import "MPMathRules.h"
 
+
+
 @implementation MPElementaryFunctionTerm {
     NSDecimalNumber *(^_function)(NSDecimalNumber *);
 }
@@ -53,16 +50,18 @@
         } else if ([function isEqualToString:@"ln"]) {
             func = &log;
         } else {
-            NSAssert(true, @"function must be one of (sin, cos, tan, asin, acos, atan, lg, log, ln).");
+            NSAssert(true, @"function must be one of (sin, cos, tan, asin, arcsin, acos, arccos, atan, arctan, lg, log, ln).");
         }
         [self setFunction:func
             takesArcValue:takesArc
           returnsArcValue:returnsArc];
         _term = term;
+        _functionIdentifier = function.copy;
     }
     return self;
 }
 
+
 - (void)setFunction:(double (*)(double))function
       takesArcValue:(BOOL)takesArc
     returnsArcValue:(BOOL)returnsArc
@@ -80,6 +79,7 @@
     };
 }
 
+
 - (NSDecimalNumber *)convertToRadiantsIfNecessary:(NSDecimalNumber *)degrees
 {
     if ([MPMathRules sharedRules].isUsingDegrees) {
@@ -90,6 +90,7 @@
     }
 }
 
+
 - (NSDecimalNumber *)convertToDegreesIfNecessary:(NSDecimalNumber *)radiants
 {
     if ([MPMathRules sharedRules].isUsingDegrees) {
@@ -100,13 +101,24 @@
     }
 }
 
+
 - (NSDecimalNumber *)doEvaluation:(NSError *__autoreleasing *)error
 {
     NSDecimalNumber *value = [self.term evaluate:error];
     if (!value) {
         return nil;
     }
-    return _function(value);
+    NSDecimalNumber *result = _function(value);
+    if ([result isEqualToNumber:[NSDecimalNumber notANumber]]) {
+        result = nil;
+        if (error) {
+            NSString *errorDescription = [NSString stringWithFormat:NSLocalizedString(@"%@ is not defined for the value %@.", nil), self.functionIdentifier, [value descriptionWithLocale:[NSLocale currentLocale]]];
+            *error = [NSError errorWithDomain:MPMathKitErrorDomain
+                                         code:102
+                                     userInfo:@{NSLocalizedDescriptionKey: errorDescription}];
+        }
+    }
+    return result;
 }
 
 @end