src/Shared/Attribute/AttributeTrait.php
public function addAttribute(/*# string */ $attrName, $attrValue)
    {
        if (!is_array($this->attributes[$attrName])) {
            $this->attributes[$attrName] = (array) $this->attributes[$attrName];
        }
        $this->attributes[$attrName][] = $attrValue;
        return $this;
    }
public function addAttribute(/*# string */ $attrName, $attrValue)
    {
        if (!is_array($this->attributes[$attrName])) {
            $this->attributes[$attrName] = (array) $this->attributes[$attrName];
        }
        $this->attributes[$attrName][] = $attrValue;
        return $this;
    }
{@inheritDoc}