Details and Options
This uses the generalized aggregation model from A New Kind of Science Chapter 7.
Neighborhood configurations are as specified by an outer totalistic cellular automaton rule.
While entering the rule number, one must make sure that "Dimension" is set to 2.
Possible forms for rule are:
{n,{2,1},{1,1}}

9neighbor totalistic rule

{n,{2,{{0,1,0},{1,1,1},{0,1,0}}},{1,1}}

5neigbbor totalistic rule

{n,{2,{{0,2,0},{2,1,2},{0,2,0}}},{1,1}}

5neighbor outer totalistic rule

The following keys can be used to specify a rule given as an association:
"TotalisticCode"

n

totalistic code

"OuterTotalisticCode"

n

outer totalistic code

"Dimension"

d

overall dimension (always 2)

"Neighborhood"

type

neigborhood

"Range"

r

range of rule

"Colors"

k

number of colors

"GrowthCases"

{
g
_{
1
}
,
g
_{
2
}
,
…
}

make a cell 1 when g_{i} of its neighbors are 1

"GrowthSurvivalCases"

{
{
g
_{
1
}
,
…
}
,
{
s
_{
1
}
,
…
}
}

1 for g_{i} neighbors; unchanged for s_{i}

"GrowthDecayCases"

{
{
g
_{
1
}
,
…
}
,
{
d
_{
1
}
,
…
}
}

1 for g_{i} neighbors; 0 for d_{i}

Possible settings for "Neighborhood" include:
5 or "VonNeumann"
9 or "Moore"
The number of possible aggregation system rules is as follows:
2D general rules

2^{512}

2D 9neighbor totalistic rules

2^{10}

2D 5neighbor totalistic rules

2^{6}

2D 5neighbor outer totalistic rules

2^{10}

2D outer totalistic rules 
2^{17}+1

The initial condition specification should be of the form aspec, {aspec,bspec} or {{{aspec_{1},off_{1}},{aspec_{2},off_{2}},…,{aspec_{n},off_{n}}},bspec} (for n>0). Each aspec must be a nonempty array of rank 2 whose elements at level 2 are integers i in the range 0≤i≤"Colors"1 ("Colors"=2 by default).
t should be a natural number. If t is specified as a list of a certain depth, then the first element of the flattened list will be taken as the input.