How to replace a function in the GridFormulaEngine library in WinForms GridControl?

//Remove the SUM function
engine.RemoveFunction("SUM");

'Remove the SUM function
engine.RemoveFunction("SUM")

//Add the SUM Function
engine.AddFunction("SUM",new GridFormulaEngine.LibraryFunction(computeMySUM));
gridControl1[3,3].Text = "= SUM(100+700)";
/// <summary>
/// Get the octal value  for the specified base10 value
/// </summary>
private string base8ToInt(string args)
{
    string s = args;
    //To match the integer in the args
    MatchCollection mc = Regex.Matches(args,@"(\d)+");
    foreach(Match m in mc)
    {
    //To convert the octal to base10 values 
    s = Regex.Replace(s,m.Value,Convert.ToInt32(m.Value ,8).ToString()); 
    }
    return s;
}
/// <summary>
/// Get the string notation for the specified range of cells
/// </summary>
private string ParseRangeOfValues(string args)
{
    //To match the cellranges
    //The group <cellRangeBegin> matches the beginning of the range.
    //The group <cellRangeEnd> matches the ending of the range.
    Match m1 = Regex.Match(args,@"(?<cellRangeBegin>((\w)+))(\s*):(\s*)(?<cellRangeEnd>((\w)+))");
    if(m1.Success)
    {
    string start = m1.Groups["cellRangeBegin"].Value;   
    string end = m1.Groups["cellRangeEnd"].Value;
    int rowFrom;
    int rowTo;
    int colFrom;
    int colTo;
    //To match the starting range for separating the column and row
    //The group <column1> matches the column name
    //The group <row1> matches the row no                          Match mrange1 = Regex.Match( start,@"(?<column1>(([a-zA-Z])))(?<row1>(\d+))");
    //To match the ending range for separating the column and row
    //The group <column1> matches the column name
    //The group <row1> matches the row no
    Match mrange2 = Regex.Match( end,@"(?<column1>(([a-zA-Z])))(?<row1>(\d+))");
    start = "";
    if(mrange1.Success && mrange2.Success)
    {
        //To convert the string row to integer row no
        rowFrom = Convert.ToInt32(mrange1.Groups["row1"].Value);  
        rowTo = Convert.ToInt32( mrange2.Groups["row1"].Value); 
        //To convert the string column to integer col no
        colFrom = mrange1.Groups["column1"].Value.ToUpper()[0] - 'A' + 1;  
        colTo = mrange2.Groups["column1"].Value.ToUpper()[0] - 'A' + 1; 
        for(int i = rowFrom ;i <= rowTo;i++)
        {
        for(int j = colFrom;j <= colTo;j++)
        {
            //To match the 8 and 9 digits to avoid the error
            if(Regex.Match(gridControl1[i,j].Text,"[89]").Success)
            return "Only Octal digits are Allowed for Sum function";
            //To convert the octal to decimal
            start += Convert.ToInt32(gridControl1[i,j].Text,8).ToString() + "+";  
        }
        }
        start += "0ab";
        start = "bn" + start;
        //To convert into required notation. 
        start = Regex.Replace(start,@"\+","an");
        //To replace the first  'an' to n 
        start = Regex.Replace(start,@"bn(?<num>(\d+))an","bn${num}n");
        return start;
    }
    }
    return "";
}
/// <summary>
/// compute the sum for specified args
/// </summary>
private string computeMySUM(string args)
{
    // to match the octal notation
    if( !Regex.Match(args,"[89]").Success )   
    {
    string s = args;
    //to match the range (:)
    Match m1 = Regex.Match(args,":");
    if( !m1.Success)
    {
         //to convert the octal to decimal
         s = this.base8ToInt(args);
    }
    else
    {
        //to parse the specified range of the cells
        s = this.ParseRangeOfValues(args);
        if(s.Equals("Only Octal digits are Allowed for Sum function"))
        return s;
    }
    string  inum = engine.ComputeSum(s);
    //to convert decimal to octal 
    return covertToBase8(int.Parse(inum));
    }
    else
    return "Only Octal digits are Allowed for Sum Function";
}
private string covertToBase8(int s)
{
    string sum = "";
    while(s >0)
    {
    sum += s % 8 ;
    s = (int)s/8;
    }
    int num = 0;
    // to reverse the sum value 
    for(int i = 0 ;i<sum.Length;i++)
    {
    num += Convert.ToInt32(sum[i].ToString()) * (int)Math.Pow(10.0,(double)i) ;
    }
    sum = num.ToString(); 
    return sum ; 
}

'Add the SUM Function
engine.AddFunction("SUM",New GridFormulaEngine.LibraryFunction(AddressOf computeMySUM))
gridControl1(3,3).Text = "= SUM(100+700)"
''' <summary>
''' Get the octal value  for the specified base10 value
''' </summary>
private String base8ToInt(String args)
    Dim s As String = args
    'To match the integer in the args
     Dim mc As MatchCollection = Regex.Matches(args,"(\d)+")
     For Each m As Match In mc
         'To convert the octal to base10 values 
         s = Regex.Replace(s,m.Value,Convert.ToInt32(m.Value,8).ToString())
     Next m
     Return s
''' <summary>
''' Get the string notation for the specified range of cells
''' </summary>
private String ParseRangeOfValues(String args)
       'To match the cellranges
       'The group <cellRangeBegin> matches the beginning of the range.
       'The group <cellRangeEnd> matches the ending of the range.
       Dim m1 As Match = Regex.Match(args,"(?<cellRangeBegin>((\w)+))(\s*):(\s*)(?<cellRangeEnd>((\w)+))")
       If m1.Success Then
         Dim start As String = m1.Groups("cellRangeBegin").Value
         Dim [end] As String = m1.Groups("cellRangeEnd").Value
         Dim rowFrom As Integer
         Dim rowTo As Integer
         Dim colFrom As Integer
         Dim colTo As Integer
         'To match the starting range for seperating the column and row
         'The group <column1> match the column name
         'The group <row1> matches the row no
         Dim mrange1 As Match = Regex.Match(start,"(?<column1>(([a-zA-Z])))(?<row1>(\d+))")
         'To match the ending range for seperating the column and row
         'The group <column1> match the column name
         'The group <row1> matches the row no
         Dim mrange2 As Match = Regex.Match([end],"(?<column1>(([a-zA-Z])))(?<row1>(\d+))")
         start = ""
         If mrange1.Success AndAlso mrange2.Success Then
             'To convert the string row to integer row no
             rowFrom = Convert.ToInt32(mrange1.Groups("row1").Value)
             rowTo = Convert.ToInt32(mrange2.Groups("row1").Value)
             'To convert the string column to integer col no
             colFrom = mrange1.Groups("column1").Value.ToUpper()(0) - AscW("A"c) + 1
             colTo = mrange2.Groups("column1").Value.ToUpper()(0) - AscW("A"c) + 1
             For i As Integer = rowFrom To rowTo
                 For j As Integer = colFrom To colTo
                     'To match 8 and 9 digits for avoiding the error
                     If Regex.Match(gridControl1(i,j).Text,"[89]").Success Then
                         Return "Only Octal digits are Allowed for Sum function"
                     End If
                     'To convert the octal to decimal
                     start &= Convert.ToInt32(gridControl1(i,j).Text,8).ToString() & "+"
                     Next j
                     Next i
            start &= "0ab"
            start = "bn" & start
            'To convert into required notation. 
            start = Regex.Replace(start,"\+","an")
            'To replace the first  'an' to n 
            start = Regex.Replace(start,"bn(?<num>(\d+))an","bn${num}n")
            Return start
        End If
    End If
    Return ""
''' <summary>
''' compute the sum for specified args
''' </summary>
private String computeMySUM(String args)
    ' to match the octal notation
    If Not Regex.Match(args,"[89]").Success Then
        Dim s As String = args
        'To match the range (:)
        Dim m1 As Match = Regex.Match(args,":")
        If Not m1.Success Then
            'to convert the octal to decimal
            s = Me.base8ToInt(args)
        Else
            'to parse the specified range of the cells
            s = Me.ParseRangeOfValues(args)
            If s.Equals("Only Octal digits are Allowed for Sum function") Then
                Return s
            End If
        End If
        Dim inum As String = engine.ComputeSum(s)
        'to convert decimal to octal 
        Return covertToBase8(Integer.Parse(inum))
    Else
        Return "Only Octal digits are Allowed for Sum Function"
    End If
private String covertToBase8(Integer s)
    Dim sum As String = ""
    Do While s >0
        sum &= s Mod 8
        s = CInt(Fix(s))/8
    Loop
    Dim num As Integer = 0
    ' to reverse the sum value 
    For i As Integer = 0 To sum.Length - 1
        num += Convert.ToInt32(sum.Chars(i).ToString()) * CInt(Fix(Math.Pow(10.0,CDbl(i))))
    Next i
    sum = num.ToString()
    Return sum