I'm trying to use the ParserFunctions library in lua for it's trunc() calculation, but Help:Extension:ParserFunctions has no documentation at all how its used other than mw.ext.ParserFunctions.expr. The most I was able to get is mw.ext.ParserFunctions.expr("trunc", value) but that gives me the error "Lua error: Expression error: Missing operand for trunc..".