How is the shortened form taku derived?

Imruʾ al-Qays: Address to Fāṭimah, Parting, and the Arrows of the Eyes (Verses 19–22)

• It is takun with its final nūn omitted
• An extra alif was added to kāna
• Takūn was made into an active participle
• The initial tāʾ was changed to yāʾ