The onReplaceClause, if present, causes the block to be executed each time the value of the variable changes -- including the assignment of its initial value.

The optional parameters provide information about what is changed: on replace oldValue [ firstIndex .. lastIndex ] = newElements { ... }

But these parameters are all optional, and in the simplest form none of these are present:

var x = 0 on replace { println("x is now: {x}") }

The oldValue parameter provides the previous value.

var x = 0 on replace oldValue { println("x was {oldValue} and is now: {x}") }

The remaining parameters are useful with sequences.

var seq = ['A', 'B', 'C', 'D', 'E', 'F'] on replace oldValue[firstIdx .. lastIdx] = newElements { 
    println("replaced {oldValue}[{firstIdx}..{lastIdx}] by {newElements} yielding {seq}") 
} 
seq[3] = '$'; 
insert '#' into seq; 
delete '$' from seq; 
delete seq[2]; 
seq[1..4] = ['X', 'Y']; 
delete seq; 

Which prints:

replaced [0..-1] by ABCDEF yielding ABCDEF
replaced ABCDEF[3..3] by $ yielding ABC$EF
replaced ABC$EF[6..5] by # yielding ABC$EF#
replaced ABC$EF#[3..3] by  yielding ABCEF#
replaced ABCEF#[2..2] by  yielding ABEF#
replaced ABEF#[1..4] by XY yielding AXY
replaced AXY[0..2] by  yielding

Note that firstIndex, lastIndex and newElements refer only to the portion of the sequence which has changed,

[To do: explain concept more. Note that it is inferior to binding]

[To do: brief intro to slices, plus cross-reference]