A vague idea formed at the back of his mind.

For the last few minutes, an idea had been forming in the back of his mind.

An idea was forming in the back of his mind.

But the ideas in it still form the core of the Russian government's alternative economic plan.

The last 20 years are represented here, but ideas formed early remain clear.

If he had been able to cross immediately then perhaps the idea might never have formed in his mind.

Suddenly, an image popped into his mind, and an idea formed.

As he watched them, an idea formed in his mind.

His ideas formed the basis of many art education programs in the 20th century.

She had to go on now, and the idea was forming in her mind.