lib/Assertion.php
function containOnly(string $type): self {
    return $this->expect($this->target, containsOnly($type));
  }
function containOnly(string $type): self {
    return $this->expect($this->target, containsOnly($type));
  }
Reports an error if the target does not contain only variables of a given type.
@param string $type The type to check.
@return self This instance.