Description: Define a function recs ( F ) on On , the class of ordinal numbers,
by transfinite recursion given a rule F which sets the next value
given all values so far. See df-rdg for more details on why this
definition is desirable. Unlike df-rdg which restricts the update rule
to use only the previous value, this version allows the update rule to use
all previous values, which is why it is described as "strong", although
it is actually more primitive. See recsfnon and recsval for the
primary contract of this definition. (Contributed by Stefan O'Rear, 18-Jan-2015)(Revised by Scott Fenton, 3-Aug-2020)