Exception when custom function name contains "let"

[BrianMacIntosh]

This exception is thrown when parsing an expression containing a custom function that contains the substring 'let'.

Call stack:
` mscorlib.dll!string.Substring(int startIndex, int length) Unknown

MathParser.dll!Mathos.Parser.MathParser.ProgrammaticallyParse(string mathExpression, bool correctExpression, bool identifyComments) Line 319 C#
MathParserTest.exe!Mathos.Parser.Test.Program.Main(string[] args) Line 16 C#
`

Test code:
MathParser parser = new MathParser(); parser.LocalFunctions.Add("notlet", (inputs) => inputs[0]); Console.WriteLine(parser.ProgrammaticallyParse("1 + notlet(2)"));