The mathematician John Horton Conway is best known for his Game of Life and the big symmetry groups, which can be used to pack oranges in 24-dimensional space. He got most satisfaction from his discovery of the surreal numbers though, constructed by playing Conway games to fill in the gaps between Cantor's ordinal numbers ω1..

Here we work out a passage from The Book of Numbers written by Conway and Guy, where Conway shoots off a chained arrow notation ai→... {ai#n} which (for chains with n>3 parameters) formulates Big numbers far beyond superpowers. To give a hint of its capacity, watch how the simplest of chained arrow expressions already increases with Ackermann-type function speed.

Conway modelled his chained arrows after Donald Knuth's up-arrows, which we've introduced before as the arrow operation a^...b {^#c} starting from multiplication at c=0, exponentiation at c=1, tetration at c=2, etc.
The principle that a superpower ^.. {#c1} equals a repeated number of preceding operations ^.. {#c} can be expanded to an arbitrary number of parameters, which is how Conway invented his main rule for chained arrows.

Conway and Guy's book doesn't tell what should happen with the single chained arrow a→b.
This is commonly interpreted to signify the powers a^b (plausible, for then all R→1 = R). But can also be defined independently as multiplication ab {0^} (via reduction to a→b→0 as usual), or as addition ab {0*} maybe.

Conway's formula for the first row of chained arrows covers superpowers and makes a separate axiom for the basic three parameters redundant. We'll support the historical interpretation that a→b equals a^b exponentiation and let this two parameter case be the initial axiom.
Lo and behold! Here come the first four parameters of the archetypical operator function called chained arrows.

• a→1 = a
• a→b1 = (a→b)a {0_^} == a... {#b1 0_^} = a**b1

• a→b→1 = a→b                a→b→c→1 = a→b→c
• a→1→c = a                  a→b→1→d = a→b
• a→b1→c1 = a→(a→b→c1)→c     a→b→c1→d1 = a→b→(a→b→c→d1)→d

We've presented Conway's notation in the same format as our operatorial functions (see bigE) to make clear that chained arrow signs → are more like parameter separators , than like operators. Normally operations are resolved in order of precedence, that is by operator size, but the evaluation of variables separated by chained arrows is the sole consequence of their position in the row, much like parameters in a function.

§3.0.2. Our single operator subscript

A natural alternative to bridge the conceptual gap between operators and functions is to create operator types with coefficients in the form of subscripts. Subscripted arrowheads are recursively enumerated operators of a type which transcends superpower arrows.
Here are the initial steps over operator counter c and type counter d of subscripted arrows with four parameters.

• a^_1;b = a^^...b {^#b}
• a^_1;^...b {^_1;#c1} = a^_1;^.. ... {^_1;#c a#b}
• a^_d1;b = a^_d;^...b {^_d;#b}
• a^_d;^...b {^_d;#c1} = a^_d;^.. ... {^_d;#c a#b}

At the very start operator subscripts are equivalent to chained arrows, but when a>2 we promptly speed away.
In the example below, we conveniently use the signs 2 and 3 in place of their representation in units 1..

This should give you an impression of the respective strength of these notations.  
Note that in our construction of subscripted majority arrows ^Rj;.. a sequence of operators with mixed subscripts such as a^1;^b does not make sense yet.

§3.0.3. Conway's row of chained arrows

After the first 4 parameters the rest of Conway's chained arrows follows easily.
Let the wildcard R substitute for a row of arrow→separated→parameters of arbitrary length n>0.

• R→1 = R
• R→1→z1 = R→1→z == R→1→1 = R
• R→y1→z1 = R→(R→y→z1)→z

Because a→b = a^b there are no restrictions on the range of values the two drop axioms apply to. And the main axiom without condition y>0 implies that R→0→z = 1 (true for exponentiation). Fits like a clockwork!
Interestingly, the right part of a row L→1→R == L→1→y→1 = L→1→y = L collapses on the position of the parameter value 1. Where only the left part L can be kept, this is a waste of expressive power or resolution.

Let's translate Conway's chained arrow notation Can to φ-function format. The right arrows allow us to order a number of variables in a sequence and are thus equivalent to comma separators inside a function. (This is not to say that arrows ↑ and → cannot help the lonely mathematician to connect ——→ his calculations on a sheet of paper.)

• Can_φ(a_i,...) {a_i#n} = a_i→... {a_i#n}

It seems a bit awkward that the last parameter z wasn't count down to 0 which is the preferred initial index for our operatorial functions. For the definition of multiple arrows however, this shows us the natural way to transcend Conway's single arrow notation.
By iterating directly over the length of his row of chained arrows we open up to higher dimensions and enter the next class {K.3} of Big number functions. In between the function classes already past by, and those still unheard of… the initial Conway super-jump is forthcoming.

John H. Conway must have hinted at a ψ-function of countable super-chained arrows by describing its first 4 values in his imaginative Book of numbers. Similar to the way Ackermann had to jump from primitive recursion {K.1}, super-chained arrows cannot be expressed by Ackermann-Conway {K.2} recursions alone.

Past the Conway super-jump the Can_ψ function addresses the length of a row of chained parameters directly. The extension to a double →→ arrow-operator we've concocted is apocryphal, yet appears to be a natural consequence of Conway's notation (where R→1 = R in {0_^} context).

• a→b→1 = a*... {a#b} = a→^.. ... {→#0 a#b 0_^}
• a→b→^..1 {→#c1} = a→^.. ... {→#c a#b 0_^}
• Can_ψ(a) = a→... {a#a} = a→a→→1
• Can_ψ(i) == { 1, 4, 3^^7625597484987,
                4→4→(4→4→(4→4→256→3)→3)→3, .. }

The second rule counts chained arrows when c=1 and from this Can_ψ follows at c=2 in the line below. Of the numbers Conway mentions in his book Can_ψ(4) = 4→4→4→4 is already physically inexpressible without resort to a function higher than superpowers.

§3.0.4. Our row of operator subscripts

By the same rules for subscripted operators ^d... we saw above, comma separated coefficients can be extended naturally right up to the end of a first row ^d,..,z...

But it's not so obvious how to handle the initial subscripts. We'll use an upload method that was (to our knowledge) first introduced by Jonathan Bowers in his Exploding Array Function. The concept that a low level parameter value is build up to seed higher parameters is one of the most powerful rules for creating Big numbers.
Follow the functions of e-init and e-step which substitute the value b for c=1 and d=1 to resurrect these parameters after they are count down. These are cases of what we call uploading.

• a^_1,1;b = a^_b;^...b {^_b;#b}
• a^_1,e1;b = a^_b,e;^...b {^_b,e;#b}

Continue to apply this kind of multiple substitution in the definition of subscripted arrow and star operations.
It is clear that the operator counter c and the following operator subscripts d,e,.. are similar uploadable iterators (with the provision that arrows ^R.. do not allow series of mixed subscripts but stars *R.. do).

We will suffix _c before the row of arrow coefficients _R to improve readability. Then to retrieve the original form with countable operators, you should substitute a^R;..b {^_R;#c} for a^c,R;b in this general formula for subscripted majority arrows.

• a^_1;b1 = a^_0;(a^_1;b) == a^_0;... {a#b1} = a... {a#b1 0_^}
• a^_1,...;b {1#k1} = a^_b,...;b {b#k}
• a^_1,...,n1,R;b {1#+k} = a^_b,...,n,R;b {b#k}
• a^_c1,R;b1 = a^_c,R;(a^_c1,R;b) == a^_c,R;... {a#b1}

With uploading we substitute higher parameters for a value accumulated in a lower parameter – that is b in case of subscripted arrows and stars. Without this mechanism it's hard to see how ordinary unnested functions can move past the Ackermann-Conway class as represented by chained arrows.
Uploading parameter values from the first row is definitely the way forward to conquer the next dimension.

Later, when we'll insert a new rule in bigI to accommodate mixed star subscripts, you'll see that the capacity lost by bigI's iterator collapse is redeployed to lift it a level higher in bigI's first row with waitor.
So if chained arrows send forth a whole row of similar iterator y destructions, this dormant capacity in the keeping might just be the thing that gracefully lifts a function of mixed arrows up to a higher heaven.