Enumerations for _Distance_constraint_list.Constraint_type (An open enumeration)
`disulfide bond` | 'Disulfide bond distance constraints'
`general distance` | 'Specific distance constraint is not known'
`hydrogen bond` | 'Hydrogen bond distance constraints'
`NOE` | 'Distance constraints determined from NOE experiments'
`NOE build-up` | 'Distance constraints determined from a series of NOE experiments with different mixing times'
`NOE not seen` | 'Distance constraints supplied when an expected NOE was not observed'
`paramagnetic relaxation` | 'Distance constraints determined from paramagnetic relaxation studies'
`ROE` | 'Distance constraints determined from ROE experiments'
`ROE build-up` | 'Distance constraints determine from a series of ROE experiments with different mixing times'
`symmetry` | 'Distance constraint enforces symmetry - for example to enforce symmetry between monomers in a multimer'