All relations in a slightly modified standard relational algebra with guarded bisimulation and first-order definable are known as "guarded relational algebra".
The brothers' skill was unequaled in their time; after their death, their guarded trade secrets were lost, and not until the 19th century were bells of comparable tuning quality cast.
The government houses victims and potential victims applying for asylum with other at-risk groups in guarded asylum centers to prevent unwanted contact with traffickers.