alter-text-propertyalter-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.