during the period", "711", "802", "499"], ["changes in estimated future development costs", "-556 ( 556 )", "-478 ( 478 )", "-297 ( 297 )"], ["revisions of previous quantity estimates", "494", "348", "311"], ["net changes in purchases and sales of minerals in place", "33", "-531 ( 531 )", "737"], ["net change in exchanges of minerals in place", "2013", "403", "2013"], ["accretion of discount", "790", "807", "417"], ["net change in income taxes", "-529 ( 529 )", "65", "-1288 ( 1288 )"], ["timing and other",