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Python script to replace characters in string efficiently, based on a specified list of coordinates
Description:    Based on a list of coordinates, the corresponding regions are masked out as X's.
                
                For example:
                            Unmasked Sequence   = ATGCAGGGGAAACATGATTCAGGAC
                            Repeat coordinate   = 5 - 8 (starting from 1 not 0!)
                            Masked Sequence     = ATGCXXXXGAAACATGATTCAGGAC