The PCASTL is an interpreted high level programming language which makes writing self-modifying code easier. Its features are:

• The "parent" keyword to access a parent node in the code syntax tree.
• The "childset" keyword to access a child node in the code syntax tree.
• The use of explicit code segments delimited with ` and '.
• Allows calls to dynamic-link libraries, shared object libraries or dynamic libraries.
• Object, array and chained list data types. They are respectively created by internal functions names, array and list.
• ANSI stdio.h interface for single-byte strings functions.
• Turing complete.

The most recent version is 3.5 and was released on 2018-03-31.
List of changes.

So, what's a syntax tree?

For example, inside the interpreter, the tree having its root in the variable fact after the following code has been executed:

fact = function(x)
{
   if (x == 1) return(1)
   return(x * fact(x - 1))
}

is:

Below you can see how childset and parent keywords can be used to navigate this tree.

> info(fact)
[node_type]     "function definition"
[nb_children]   2
[parameters][0] "x"
>
> info(fact.childset[0])
[node_type]     "list"
[nb_children]   1
>
> info(fact.childset[0].childset[0])
[node_type]     "variable"
[nb_children]   0
[name]  "x"
>
> info(fact.childset[1])
[node_type]     "list"
[nb_children]   2
>
> info(fact.childset[1].childset[0])
[node_type]     "if statement"
[nb_children]   2
>
> info(fact.childset[1].childset[1])
[node_type]     "function call"
[nb_children]   2
[nb_args]       1
>
> info(fact.childset[0].parent)
[node_type]     "function definition"
[nb_children]   2
[parameters][0] "x"

Here are ways to edit the tree:

> mknode(fact.childset[1], `if (x < 1) {
+    print("Parameter must be greater than zero.")
+    abort()
+ }', 0)
> fact(-2)
        "Parameter must be greater than zero."
> info(fact.childset[1])
[node_type]     "list"
[nb_children]   3
>
> info(fact.childset[1].childset[0].childset[1].childset[1])
[node_type]     "function call"
[nb_children]   2
[nb_args]       0
>
> fact.childset[1].childset[0].childset[1].childset[1] = `return(-1)'
        0x431c20
> info(fact.childset[1].childset[0].childset[1].childset[1])
[node_type]     "function call"
[nb_children]   2
[nb_args]       1
>
> fact(-2)
        "Parameter must be greater than zero."
        -1