select oxygen or nitrogen
define polar_atoms  selected
echo polar_atoms is oxygen or nitrogen

select carbon or *.P or sulphur
define unpolar_atoms  selected
echo unpolar_atoms is carbon, *.P, sulphur


select dna and ( *:I or *:J )
define dna chains I, J  selected
echo dna chains I, J

select   polar_atoms and dnk and within(3.5, (protein and polar_atoms))
define polar_dna  selected
echo polar_dna

select  unpolar_atoms and dna and within(4.5, (protein and unpolar_atoms))
define unpolar_dna  selected
echo unpolar_dna


select *.C1* or *.C2* or *.C3* or *.C4* or *.C5* or *.O4*
echo riboza

select *.P or *.O1P or *.O2P or *.O3* or *.O5*
echo fosfat


select  (*.N1 or *.C6 or *.C5 or *.C4 or *.N3 or *.C2 or *.O2 or *.N4 or *.C7 or *.O4 or *.N9 or *.C8 or *.N7  or *.N6 or *.N2 or *.O6) and (purine or  pyrimidine)
echo ao