historyFuncA UserFunc of kind = historyFunc returns values from a
<History>.
Currently only Tensor Histories can be probed by historyFunc; see Tensor Histories.
A UserFunc of kind = historyFunc is a function that takes a
single argument of a single integer, and returns a (scalar) value
from a <History> record. The argument is meant to be the
difference between the current time step and a past time step.
For example, a kind = historyFunc function will return its
most recent record given an argument of 0; it will return its
next-most-recent record given argument of 1, and so on.
history (string, required)The name of a <History> from which to retrieve data.
index (vector of integers, required)The array-index of the desired element in the History. For
example, if the History stores records that are \(3\times
2\) arrays (or tensors), then index might be [0, 0] or
[2,1]. For a History that stores scalar records, this
should simply be [0]. (N.B. the array dimensions of a
history can be looked up in the History dump-file, keeping in
mind that the first dimension is the record-index; for example,
a scalar history stores a dataset of dimensions \(n\times
1\).)