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Surrogate-model based reliability

Hello everyone,
I used PCK to build a surrogate model for an expensive finite element model. i need to conduct reliability analysis based on the developed surrogate model. I am having troubles defining the limit state function in UQlab. my limit state functions is simply as follows:
LS= Y(pck)-X(:,7)
where Y(pck) is the surrogate model output
and X(:,7) is the 7th input parameters.
Thanks in advance