Denver’s Sister Grotto, a.k.a. Madeline Johnston, crafts “delicate, almost mantra-like, anthems for the human condition,” but something about her latest half-hour-long work, “PIETA,” reminds me more of a gentle spirit’s manifesto than a mortal’s.The song evolves into and out of a variety of moments, but never builds in a conventional form, suggesting the continuous presence of an enigmatic, yet strangely warm and comforting form that, throughout its numerous changes, maintains a certain je ne sais quoi. Think of a lengthy immersive techno mix, sans hyper-masculine compulsion, and you get a small idea of the ineffable vibe Sister Grotto creates with her singular music.