Expression turning red in grasshopper

Hi I’m following a tutorial and type in a an expression on the ‘series’ c value expression : c/2

C–>x

It’s one of many Grasshopper secrets. Expressions on inputs or outputs always refer to themselves as ‘x’, regardless of their name. This is different from Expression and Evaluate components where the internal expression must use the input names. Quirky, eh? So charming.