Answer the value associated with key in table. If key is not present, answer the result of invoking the thunk default-thunk, which signals an error instead by default.
hash-table-ref/defaultis a variant that requires a third argument, default, and answers default itself instead of invoking it.
Remove the association of key in table, if present. If absent, do nothing.
Replace key's associated value in table by invoking modifier with one argument, the old value.
If key is not present, and default-thunk is provided, invoke it with no arguments to get the “old value” to be passed to modifier as above. If default-thunk is not provided in such a case, signal an error.
hash-table-update!/defaultis a variant that requires the fourth argument, which is used directly as the “old value” rather than as a thunk to be invoked to retrieve the “old value”.