alter-text-property is a compiled Lisp function in `indent.el'.

(alter-text-property FROM TO PROP FUNC &optional OBJECT)

Programmatically change value of a text-property.
For each region between FROM and TO that has a single value for PROPERTY,
apply FUNCTION to that value and sets the property to the function's result.
Optional fifth argument OBJECT specifies the string or buffer to operate on.