Compound assignment

Compound assignment combines a read, an operation, and a write:

local total = 10
total += 5
total *= 2

rc.3 supports these exact forms:

Form	Underlying operation
+=	numeric +
-=	numeric -
*=	numeric *
/=	numeric /
%=	integer %
..=	string ..

local message = "Hello"
message ..= ", Pop"

The target and right side must have the exact same numeric type, or both be String for ..=. The operation preserves the ordinary overflow, division, remainder, allocation, and trap rules.

Targets

Mutable locals and captures, mutable declared class fields, and array elements are supported:

counter.value += 1
values[index] *= scale

Parameters, numeric for bindings, record fields, and other immutable places remain invalid. Table entries support ordinary assignment in rc.3, but not compound assignment.

For an indexed target, Pop evaluates the array and index once, loads the current element, evaluates the right side, performs the operation, and stores only after those steps succeed. An invalid index traps before the right side runs.