Invert logic for better readability
Invert logic for better readability

Avoids a double negation ("unless is_invalid..."), so should be easier to read.

run-project/stringer,milligramme/stringer,InPermutation/stringer,swanson/stringer,dperson/stringer,Koronen/stringer,run-project/stringer,geemus/stringer,jqr/stringer,dperson/stringer,InPermutation/stringer,dperson/stringer,lodestone/stringer,nibble/stringer,mpraglowski/stringer,lodestone/stringer,adscott/stringer,geemus/stringer,geemus/stringer,adscott/stringer,nobuti/stringer,milligramme/stringer,thiagosil/stringer,milandobrota/stringer,jeroenj/stringer,dperson/stringer,nibble/stringer,jeroenj/stringer,mpraglowski/stringer,nobuti/stringer,dandorman/stringer,jeroenj/stringer,milandobrota/stringer,lodestone/stringer,nobuti/stringer,geemus/stringer,thiagosil/stringer,adscott/stringer,wwwjfy/stringer,jeroenj/stringer,mpraglowski/stringer,swanson/stringer,swanson/stringer,Koronen/stringer,jqr/stringer,nibble/stringer,dandorman/stringer,milligramme/stringer,Koronen/stringer,dandorman/stringer,bkbabydp/stringer,milandobrota/stringer,bkbabydp/stringer,nibble/stringer,Koronen/stringer,swanson/stringer,run-project/stringer,jqr/stringer,adscott/stringer,thiagosil/stringer,InPermutation/stringer,jqr/stringer,wwwjfy/stringer