-- mget-or m k default: value at key k, or default if missing.
-- lget-or xs i default: element at index i, or default if OOB.
-- Both return the element type directly (not O v), no `??` ceremony.
scores>n;m=mmap;m=mset m "alice" 99;m=mset m "bob" 87;mget-or m "alice" 0
missing>n;m=mmap;m=mset m "alice" 99;mget-or m "carol" 0
third xs:L n>n;lget-or xs 2 -1
last xs:L n>n;lget-or xs -1 -1
oob xs:L n>n;lget-or xs 99 -1
neg-oob xs:L n>n;lget-or xs -99 -1
-- run: scores
-- out: 99
-- run: missing
-- out: 0
-- run: third [10,20,30]
-- out: 30
-- run: third [10]
-- out: -1
-- run: last [10,20,30]
-- out: 30
-- run: oob [10,20,30]
-- out: -1
-- run: neg-oob [10,20,30]
-- out: -1