OpenEuphoria

Where is this in the manual defined?

For a number of procedures or functions--see below--some parameters may have the same value in many cases. The most expected value for any parameter may be given a default value. Omitting the value of such a parameter on a given call will cause its default value to be passed.

procedure foo(sequence s, integer n=1) 
    ? n + length(s) 
end procedure 
 
foo("abc")    -- prints out 4 = 3 + 1. n was not specified, so was set to 1. 
foo("abc", 3) -- prints out 6 = 3 + 3