See:
http://msdn.microsoft.com/en-us/library/txafckwd.aspx[
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Format Item Syntax
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Each format item takes the following form and consists of the following components:
{ index[,alignment][:formatString]}
The matching braces ("{" and "}") are required.
Alignment Component
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The optional alignment component is a signed integer indicating the preferred formatted field width. If the value of alignment is less than the length of the formatted string, alignment is ignored and the length of the formatted string is used as the field width. The formatted data in the field is right-aligned if alignment is positive and left-aligned if alignment is negative. If padding is necessary, white space is used. The comma is required if alignment is specified.