first

first picks the earliest available value. For example, given the following case fragment:

           01 Jun 02    30 Jun 03     02 Nov 03
HDL           2.0          2.2           1.9
LDL                        4.5           3.1
LipidDrug                                Y

first HDL is 2.0

first LDL is 4.5

( first ( HDL where ( LDL is available ) ) ) is 2.1