I. strands at the edge of sheet
1. read the Richardson's paper.
2 . Statistical analysis of hydrophobic and hydrophylic residues in strands at the edge of a sheet
(strand with many hydrophilic residues is preferable at the edge)
3. Long strand usually not at the edge of sheet ?
... many other Mark's ideas

II. strands in a strandon
1. Check and compare the length of loops between strands within a strandon and between strandons.
2. check H-bonds between strands in strandons: any relations (loop, sequence in strands, and something which we don 't know else) with H-bonds between hydrophobic or hydrophilic a.a.