src/Verification/Cardinality.php
public function assertSingular(): self
    {
        if ($this->minimum > 1 || $this->maximum > 1 || $this->isAlways) {
            throw new InvalidSingularCardinalityException($this);
        }

        return $this;
    }
public function assertSingular(): self
    {
        if ($this->minimum > 1 || $this->maximum > 1 || $this->isAlways) {
            throw new InvalidSingularCardinalityException($this);
        }

        return $this;
    }
public function assertSingular ( ) : self { if ( $ this -> minimum > 1 || $ this -> maximum > 1 || $ this -> isAlways ) { throw new InvalidSingularCardinalityException ( $ this ) ; } return $ this ; }
Asserts that this cardinality is suitable for events that can only happen
once or not at all.

@return $this                       This cardinality.
@throws InvalidCardinalityException If the cardinality is invalid.
Asserts that this cardinality is suitable for events that can only happen once or not at all .