'map' uses prohibitive amount of memory when K=20

Hi,

Thank you for providing a useful tool. I have got an issue though while computing (20, 2)-mappability with the prebuilt index of human genome (grch38-no-alt). It uses up the entire 500GB of memory on my ec2 instance and the process get killed. Is this something too much to ask for the tool? I'd appreciate any comment. Thanks.