why can .SECONDARY change build order? (fwd)

[Martin d'Anjou]

Hi,

The prerequisite build order does not necessarily match the explicit order 
on the target line. I have a very simplistic example to show that 
.SECONDARY changes the build order:

all: prereq1 prereq2

prereq1:
        @echo 1

prereq2:
        @echo 2

.SECONDARY: prereq1

Run with and without .SECONDARY (comment it out), and the order changes 
(1-2 and 2-1). I don't understand why. Can someone explain?

Thanks,
Martin d'Anjou