Function runnable_of 

pub fn runnable_of(line: &str, ctx: Context<'_>) -> Step

How an interface takes a line speccoder printed — a fix or a next step: performed through perform, a view, the person’s own command, or a line for their shell. The one parser every builder’s lines go through (in their serialisation). How line is taken, with ctx filling what it leaves open.

A line is text, and text can carry values spliced into it (a file’s name, a feature’s): so a line becomes the person’s own command (How::Command) only when it is one of speccoder’s constant lines — a catalog fix or a builder’s line, its {task}/{feature} slots filled with plain identifiers. A source that wants a command with other values gives its argvs (person, steps_with). A speccoder command that speccoder performs or opens is read from any line: what it becomes is a typed runnable, never an argv.