Given the task definition and input, reply with output. In this task, you're given passages that contain mentions of names of people, places, or things. Some of these mentions refer to the same person, place, or thing. Your job is to write questions that evaluate one's understanding of such references. Good questions are expected to link pronouns (she, her, him, his, their, etc.) or other mentions to people, places, or things to which they may refer. Do not ask questions that can be answered correctly without understanding the paragraph or having multiple answers. Avoid questions that do not link phrases referring to the same entity. For each of your questions, the answer should be one or more phrases in the paragraph, and it should be unambiguous.

Passage: The Annunciation draws heavily on van der Weyden's 1430s Louvre Annunciation, his c. 1455  Saint Columba altarpiece, and the Clugny Annunciation (c. 1465–75), which is attributed to either van der Weyden or Memling. Memling almost certainly was apprenticed to van der Weyden in Brussels until setting up his own workshop in Bruges sometime after 1465. Memling's Annunciation is more innovative, with motifs such as the attendant angels that were absent in the earlier paintings. According to Till-Holger Borchert, not only was Memling familiar with van der Weyden's motifs and compositions, but he might have assisted with the underdrawing in van der Weyden's workshop. The shutters on the right are copied from the Louvre panel, the knotted curtain appears in the Columba triptych's "Annunciation".A sense of movement is conveyed throughout. The trailing edges of Gabriel's garment fall outside the pictorial space, indicating his arrival. The Virgin's "serpentine" pose, with attendant angels supporting her, adds to the sense of flow. Memling's use of color achieves a startling effect. The traditional rays of light are replaced with light color indicators; the white clothes rendered in "icy" blue, the angel to the right in yellow patches seems "bleached by light", the left-hand angel appears to be steeped in shadow, dressed in clothes of lavender and bearing deep green wings. The effect is iridescent, according to Blum, who writes, "this shimmering surface gives [the figures] an unearthly quality, separating them from the more believable world of the bedchamber." The effect deviates from the pure naturalism and realism which typifies Early Netherlandish art, causing a "startling" juxtaposition, an effect that is "unsteadying" and contradictory.
What is the name of the person who might have assisted with the underdrawing in Van der Weyden's workshop?