How to dig in `ColumnDefinition::nullable` vendor method?

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`$table->string('example')->nullable()->unique();`
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<?php

namespace Illuminate\Database\Schema;

use Illuminate\Support\Fluent;

/**
 * @method $this after(string $column) Place the column "after" another column (MySQL)
 * @method $this always(bool $value = true) Used as a modifier for generatedAs() (PostgreSQL)
 * @method $this autoIncrement() Set INTEGER columns as auto-increment (primary key)
 * @method $this change() Change the column
 * @method $this charset(string $charset) Specify a character set for the column (MySQL)
 * @method $this collation(string $collation) Specify a collation for the column (MySQL/PostgreSQL/SQL Server)
 * @method $this comment(string $comment) Add a comment to the column (MySQL/PostgreSQL)
 * @method $this default(mixed $value) Specify a "default" value for the column
 * @method $this first() Place the column "first" in the table (MySQL)
 * @method $this from(int $startingValue) Set the starting value of an auto-incrementing field (MySQL / PostgreSQL)
 * @method $this generatedAs(string|Expression $expression = null) Create a SQL compliant identity column (PostgreSQL)
 * @method $this index(string $indexName = null) Add an index
 * @method $this invisible() Specify that the column should be invisible to "SELECT *" (MySQL)
 * @method $this nullable(bool $value = true) Allow NULL values to be inserted into the column
 * @method $this persisted() Mark the computed generated column as persistent (SQL Server)
 ...
 */
class ColumnDefinition extends Fluent
{
    //
}