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Level

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Level[expr, levelspec]

gives a list of all subexpressions of expr at the
level(s) specified by levelspec.

Level uses standard level specifications:

n

levels 1 through n

Infinity

all levels from level 1

{n}

level n only

{m, n}

levels m through n

Level 0 corresponds to the whole expression.

A negative level -n consists of parts with depth n.

Level -1 is the set of atoms in an expression:

• • Level[a + b ^ 3 * f[2 x ^ 2], {-1}]

• • Level[{{{{a}}}}, 3]

• • Level[{{{{a}}}}, -4]

• • Level[{{{{a}}}}, -5]

• • Level[h0[h1[h2[h3[a]]]], {0, -1}]

Use the option Heads -> True to include heads:

• • Level[{{{{a}}}}, 3, Heads -> True]

• • Level[x^2 + y^3, 3, Heads -> True]

• • Level[a ^ 2 + 2 * b, {-1}, Heads -> True]

• • Level[f[g[h]][x], {-1}, Heads -> True]

• • Level[f[g[h]][x], {-2, -1}, Heads -> True]