Removed Prefix/Suffix Placeholder (not needed)

Added Operator Constants
Removed NSMutableString from NSMutableExpression (not needed)
Implemented -description
Added Tests for: -description, -copy, -mutableCopy
Some Logical Corrections.

MathPadTests/MPExpressionTests.m

@@ -154,7 +154,62 @@
     XCTAssertEqualObjects(expression2, expression3);
 }
 
-// TODO: Test prefix- and suffix operators via -description method
+- (void)testDescription {
+    // Test Simple Expressions
+    MPExpression *testExpression = [[MPExpression alloc] initWithString:@"1234"];
+    XCTAssertEqualObjects([testExpression description], @"1234");
+    
+    testExpression = [[MPExpression alloc] initWithFunction:[[MPFunction alloc] init]];
+    XCTAssertEqualObjects([testExpression description], @"[]");
+    
+    // Test function after literal without explicit operator
+    testExpression = [[MPExpression alloc] initWithSymbols:@[@"123", [[MPFunction alloc] init]]];
+    XCTAssertEqualObjects([testExpression description], @"123*[]");
+    
+    // Test function after literal with explicit operator
+    testExpression = [[MPExpression alloc] initWithSymbols:@[@"123+", [[MPFunction alloc] init]]];
+    XCTAssertEqualObjects([testExpression description], @"123+[]");
+    
+    // Test literal after function without explicit operator
+    testExpression = [[MPExpression alloc] initWithSymbols:@[[[MPFunction alloc] init], @"123"]];
+    XCTAssertEqualObjects([testExpression description], @"[]*123");
+    
+    // Test literal after function with explicit operator
+    testExpression = [[MPExpression alloc] initWithSymbols:@[[[MPFunction alloc] init], @"-123"]];
+    XCTAssertEqualObjects([testExpression description], @"[]-123");
+    
+    // Test function after function without explicit operator
+    testExpression = [[MPExpression alloc] initWithSymbols:@[[[MPFunction alloc] init], [[MPFunction alloc] init]]];
+    XCTAssertEqualObjects([testExpression description], @"[]*[]");
+    
+    // Test function after function with explicit operator
+    testExpression = [[MPExpression alloc] initWithSymbols:@[[[MPFunction alloc] init], @"-", [[MPFunction alloc] init]]];
+    XCTAssertEqualObjects([testExpression description], @"[]-[]");
+    
+    
+    // Test whitespaces in literal
+    testExpression = [[MPExpression alloc] initWithSymbols:@[@" 123 + ", [[MPFunction alloc] init]]];
+    XCTAssertEqualObjects([testExpression description], @"123 +[]");
+}
+
+- (void)testCopying {
+    MPExpression *baseExpression = [[MPExpression alloc] initWithFunction:[[MPFunction alloc] init]];
+    MPExpression *copy = [baseExpression copy];
+    
+    XCTAssertNotEqual(copy, baseExpression);
+    XCTAssertNotEqual([baseExpression symbolAtIndex:0], [copy symbolAtIndex:0]);
+}
+
+- (void)testMutableCopying {
+    MPExpression *baseExpression = [[MPExpression alloc] initWithString:@"123"];
+    MPMutableExpression *expression1 = [baseExpression mutableCopy];
+    [expression1 appendFunction:[[MPFunction alloc] init]];
+    MPExpression *expression2 = [[MPExpression alloc] initWithSymbols:@[@"123", [[MPFunction alloc] init]]];
+    
+    XCTAssertEqualObjects(expression1, expression2);
+    XCTAssertNotEqualObjects(baseExpression, expression1);
+}
+
 // TODO: Test evaluating expressions
 
 @end