I noticed that the inner bank of the curve...

Was of white marble, and so decorated
With carvings that not only Polycletus
But nature herself would there be put to shame...

There was recorded the high glory
Of that ruler of Rome whose worth
Moved Gregory to his great victory;

I mean by this the Emperor Trajan;
And at his bridle a poor widow
Whose attitude bespoke tears and grief...

The wretched woman, in the midst of all this,
Seemed to be saying: 'Lord, avenge my son,
Who is dead, so that my heart is broken..'

So he said: 'Now be comforted, for I must
Carry out my duty before I go on:
Justice requires it and pity holds me back.'

Dante, The Divine Comedy, Purgatorio X, ll. 32 f. and 73 f.