start_mapping

        # Psych::TreeBuilder.start_mapping

(from ruby core)
### Implementation from Handler
---
    start_mapping(anchor, tag, implicit, style)

---

Called when a map starts.

`anchor` is the anchor associated with the map or `nil`. `tag` is the
tag associated with the map or `nil`. `implicit` is a boolean indicating
whether or not the map was implicitly started. `style` is an integer
indicating the mapping style.

See the constants in Psych::Nodes::Mapping for the possible values of
`style`.

### Example

Here is a YAML document that exercises most of the possible ways this
method can be called:

    ---
    k: !!map { hello: world }
    v: &pewpew
      hello: world

The above YAML document consists of three maps, an outer map that
contains two inner maps.  Below is a matrix of the parameters sent in
order to represent these three maps:

    # anchor    tag                       implicit  style
    [nil,       nil,                      true,     1     ]
    [nil,       "tag:yaml.org,2002:map",  false,    2     ]
    ["pewpew",  nil,                      true,     1     ]