Checks if a value is a Model single-value template.
Accepts a single Placeholder, a Union per-branch placeholder, or a Locales localised text
map: the single-value forms admitted as Placeholders alongside the collection-valued Query, and as
the per-cell value of a Projection.
Checks if a value is a Model single-value template.
Accepts a single Placeholder, a Union per-branch placeholder, or a Locales localised text map: the single-value forms admitted as Placeholders alongside the collection-valued Query, and as the per-cell value of a Projection.