For functions that are mathematically associative (and possibly commutative), a conforming implementation may process the arguments in any manner consistent with associative (and possibly commutative) rearrangement. This does not affect the order in which the argument forms are evaluated; for a discussion of evaluation order, see Function Forms. What is unspecified is only the order in which the parameter values are processed. This implies that implementations may differ in which automatic coercions are applied; see Contagion in Numeric Operations.
A conforming program can control the order of processing explicitly by separating the operations into separate (possibly nested) function forms, or by writing explicit calls to functions that perform coercions.