{{/*
  Param substring shortcode: psubstr PARAM_NAME START
*/}}
{{- $name := (.Get 0) -}}
{{ $start := (.Get 1) -}}
{{ with $name -}}
  {{ with ($.Page.Param .) -}}
    {{ substr . $start -}}
  {{ else -}}
    {{ errorf "Param %q not found: %s" $name $.Position -}}
  {{ end -}}
{{ else -}}
  {{ errorf "Missing param key: %s" $.Position -}}
{{ end -}}
