A sum block allows to add N input signals together to produce a single output signal. Each of the input signals can be negated if necessary with

void negateInput(uint8_t index);

Situations can arise that a sum blocks draws one of its inputs from signals which are produced further down in the chain, e.g. feedback signals.

Feedback signal in a sum block

In the first run the feedback signal generally has no value yet (is nan) and its timestamp is not set. The output of the sum block would be nan as well. In such cases you can preset the signal of an input to the sum block with

void setInitCondition(uint8_t index, T val);