Document the intention behind mps_ld_isstale rather than the implementation: it captures the intention of testing whether a particular dependency is stale (even though the implementation in fact tests whether any dependency is stale). Add new function mps_ls_isstale_any for expressing the intention of testing whether any dependencey is stale. The two functions have  the same implementation at present, but the separation gives us the freedom to return a more precise result from mps_ld_isstale in future.