@param fn BAM/BCF/etc filename, to which .bai/.csi/etc will be added or the extension substituted, to search for an existing index file @param fmt One of the HTS_FMT_* index formats @return The index, or NULL if an error occurred.
See Implementation
@param fn BAM/BCF/etc filename, to which .bai/.csi/etc will be added or the extension substituted, to search for an existing index file @param fmt One of the HTS_FMT_* index formats @return The index, or NULL if an error occurred.