Function render_expr 

pub fn render_expr(
    arena: &ExprArena,
    id: ExprId,
    resolve: &impl Fn(VarId) -> String,
) -> String

Render id as a canonical infix string, resolving each VarId to a display name via resolve. The linear part first, then any nonlinear residual summands. Signs are folded into the joins, and a value-less expression is rendered as 0.

Parameters render as their current arena value.