Expression Syntax

Hi,

how is the right expression syntax if i want to control only one value of a value pair (y position only) ?

I tried this :

xpos;

ypos = comp("Concept_01").layer("Control").effect("Y-Pos Sub-Title")("Slider");

[xpos, ypos]

but that gives an error and i don't know how to "not define" the x-pos.

Regards,

Frank