some renaming of variables

specs/in_review/S18_new-rules-for-plotting-dance-groups-in-show.md

@@ -16,11 +16,11 @@ See [validation evidence](../../docs/s18-validation.md) and
 
 - A show consists of dayparts and can span multiple dates, typically two days.
   All counts below apply across the entire show, not separately per date.
-- Each show has one configurable integer `x`, between 1 and the number of
+- Each show has one configurable integer `preferredDayParts`, between 1 and the number of
   dayparts inclusive. Its default is 2, or 1 for a one-daypart show.
 - This replaces the existing configurable appearance counts per group type.
 - A show without dayparts can be stored but cannot be scheduled.
-- If reducing the number of dayparts makes `x` invalid, require the user to
+- If reducing the number of dayparts makes `preferredDayParts` invalid, require the user to
   correct it before scheduling again; do not silently change it.
 - Groups without dancers are excluded from scheduling and from all performance
   counts and showcase-position requirements.
@@ -28,9 +28,9 @@ See [validation evidence](../../docs/s18-validation.md) and
 ## Required group appearances
 
 - Each nonempty REGULAR, SELECTION and PRE_EDUCATION group performs in exactly
-  `x` distinct dayparts, once within each selected daypart.
+  `preferredDayParts` distinct dayparts, once within each selected daypart.
 - Each nonempty PRE_SCHOOLER group performs exactly once in the complete show,
-  regardless of `x`. At most two such groups are expected. They can be assigned
+  regardless of `preferredDayParts`. At most two such groups are expected. They can be assigned
   freely to dayparts; no additional placement restriction applies.
 - Each nonempty SHOW_CASE group performs exactly once in every daypart.
 - With at least two showcase groups, each daypart starts with one showcase and
@@ -42,12 +42,12 @@ See [validation evidence](../../docs/s18-validation.md) and
 
 ## Dancer participation and coaches
 
-- A dancer without a showcase membership performs in at most `x` distinct
+- A dancer without a showcase membership performs in at most `preferredDayParts` distinct
   dayparts across the complete show. This is a hard constraint.
 - A showcase participant necessarily performs in all dayparts. Their other
-  group performances may span more than `x` dayparts, but this is undesirable.
+  group performances may span more than `preferredDayParts` dayparts, but this is undesirable.
 - For each non-coach showcase participant, count the distinct dayparts in which
-  they perform outside showcases. Their excess is `max(0, count - x)`.
+  they perform outside showcases. Their excess is `max(0, count - preferredDayParts)`.
   Minimize the sum of these excesses as the first optimization priority.
 - A person dancing in several groups must use the same dancer registration in
   every group, including when accompanying preschoolers. Up to five group