What is the sum of total pounds of purchase in year 2018 for all branches in London?
return branches ;return #1 in London ;return purchases of #2 ;return #3 in 2018 ;return pounds of #4 ;return the  sum of #5 for each #2 ;return the  sum of #6
['select', 'filter', 'project', 'filter', 'project', 'group', 'aggregate']