I was reading "Intuitionism - an Introduction, A. Heyting" but I could not understand his definitions of "species" and "spreads" (and also there is a thing which he calls "ips"?).
As I understand, "species" and "spreads" are two different types of sets, but I am having a little bit trouble following the book.
Is there anyone who are -or willing to be- familiar with Heyting's book, and can say something about these notions?
As an example, book says:
3.1.2. Definition of a Spread
A $spread$ $M$ is defined by two laws; the first, which I shall call the $spread-law$ $\Lambda_M$, regulates the choices of natural numbers, while the effect of the second or $complementary$ $law$ $\Gamma_M$ is to assign a sequence of mathematical entities to any ips of natural numbers which is generated according to the first law.