# `?` — Predicate

Syntax

In a dfn, `C?` ends a predicate. When `C` is true, the rest of the body
runs. When it’s false, the dfn moves on to its next body, after `;`.

``` bpl
{⍵<0?-⍵;⍵}¯3      ⍝ 3
{⍵<0?-⍵;⍵}4       ⍝ 4
```

Statements before a predicate run first. After it, a body can run
several statements, or hold more predicates.

``` bpl
{⍵<0?a←-⍵ ⋄ a×2;⍵}¯3   ⍝ 6
{⍵>0?⍵<10?⍵;0}15       ⍝ 0
```

`C` must be a Boolean singleton. A body that ends after its predicate
returns no result. If every predicate fails, the call is a `DOMAIN`
error. See [Dfns](../dfns.ipynb).